Thursday, March 5, 2015

Aquarium Fish Feeding

Food must provide fish with the elements needed to "build" their body (proteins) and the energy (from proteins, carbohydrates, and lipids) required for its smooth functioning.
Feeding a fish properly ensures that it will grow satisfactorily, as well as facilitating its reproduction and helping it to combat disease. Quality and quantity are two important concepts in feeding: an aquarist must learn to avoid over-feeding and to diversify the food supply.


With regard to fishes' feeding habits and dietary requirements, there are three main groups:

- Carnivores, which feed on worms, crustaceans, insects, or other fish;
- Herbivores, with a diet mainly comprising vegetable matter (plants, algae) which they graze or grind;
- Omnivores, which have a very varied diet as they eat both animal prey and vegetable matter. In practice, diets are not always so easy to define. In a natural setting fish eat what they find, and so sometimes a herbivore will eat a small animal taking shelter in the plants that it normally eats.

Carnivorous fish mainly require proteins and lipids, while herbivores have a special need for carbohydrates, and omnivores demand a mixture of proteins, carbohydrates, and lipids.
Bearing in mind the limitations of these definitions, all three diets can be found among those given to aquarium fish, although there are some exceptions. The aquarist should also take care that vitamins and mineral salts form part of a varied and balanced diet, especially through the addition of fresh or live food.

A wide palette of foodstuffs stimulates the growth and reproduction of the aquarium's inhabitants and strengthens their resistance against disease. As such, it is the best precaution that can be taken against disappointing results and lack of success.


There is a tendency to overfeed aquarium fish and produce some rather flabby specimens. Moreover, there is an increased risk of pollution in a confined space: the more a fish eats, the more it excretes nitrogenous substances, and that is without counting the foodstuffs that quickly decompose in the water in the aquarium.

An adult fish only eats 1 or 2% of its own body weight per day, although a juvenile consumes twice that amount. Very light flakes involve little risk of overdosing, in contrast with other foodstuffs which can sometimes slip out of the hand. Whenever possible, it is advisable to divide the daily input into separate portions, twice a day for adults, more often for fry.

As fish in their natural habitat are unlikely to eat regularly every day, most aquarium fish will therefore cope well with a short term fast. Some aquarists impose a one-day fast per week to compensate for the likelihood that the fish have been overfed on the other days. Fasting is however not suitable for fry as it may slow their growth considerably.


Flakes of vegetable matter
This is dry food which is commercially widely available in specialist aquarium stores. In the last twenty years the entire range has diversified considerably, and today there is a wide variety, adapted to the needs of different groups of fishes: for juvenile and adult fish, for freshwater and seawater fish, etc. These foodstuffs are characterized by a high level of proteins (generally 40-50%) and come in different forms: in flakes (the most common), granulated, or compressed.

The flakes float for a while before they sink, which makes them easier to grasp for surface and openwater fish. There are obviously also different sizes of foodstuffs, according to the size of the fish's mouth. This artificial food is fragile and deteriorates if it is not kept in the correct conditions. It must therefore be stored in a dry place protected from the light. Its composition is only guaranteed for a certain period, so it is advisable to buy a small box if
flakes for adult heroes
you have only one aquarium and a few fish. Some aquarists do not hesitate to give their residents trout food, which they buy at fish farms. This food is very rich in proteins and lipids, thus ensuring the rapid growth of trout bred for eating, but this not vital in aquariums.
flakes of guppies
Although such food contains pigments intended to change the flesh color of salmon fish, aquarists who have used it have not reported any modifications in the external color of their fish. In any case, this food can prove very economical for large-scale breeding or garden ponds.

Marine fish sometimes refuse artificial food, either for a short period after their introduction to the aquarium, or on a permanent basis. One trick is to progressively incorporate increasing amounts of commercial food along with fresh food or live prey. This gradually accustoms the fish to its smell and taste until they finally accept it on its own.


The artificial foodstuffs currently on the market are very effective for aquarium fish. In order to cover their needs as fully as possible, fish can also be served small, live prey, which are similarly available commercially and are an important source of vitamins and mineral salts. There are other possible options: white meat and mussels, for example.


Aquarium Freeze-dried food
It is possible to buy freeze-dried food items - small animals, shrimps, worms, or plankton - in which the water content has been greatly reduced so that they can be preserved more effectively. This treatment does clearly make them expensive, but they are at least as nourishing as flakes and very popular with fish. They must be stored in a dry place. Feed them to the fish according to the manufacturer's directions and do not exceed the quantities recommended.

Aquarium feeding with Frozen food
Frozen food:
crab and
Frozen products - shrimps, fish, worms, and plankton - can also be used, once they have been separated and rinsed. They have a very high nutritional value, as freezing does not modify their composition. They are obviously stored in the freezer and must not be refrozen after thawing, in order to reduce the risk of microbial contamination. They are more expensive than freeze-dried foodstuffs.

Aquarium feeding with Domestic food items
Finally, domestic food can be provided fresh or after freezing and thawing. It is best to avoid red meats as they have too much fat. Beef heart, rich in both blood and lipids, can only be given only to large fish. White meats are preferable: chicken, turkey, or ham. As for seafood, white fish can be used - although it must be handled with care, as it can break up in the water - as well as mussels, cockles, and shrimps, which can be bought preserved naturally. Vegetables are sometimes needed for herbivorous fish: lettuce or spinach that has been blanched, i.e

Aquarium feeding with Live Prey
These are ideal food items for carnivorous fish: they retain all their nutritional value and move around to attract the fish. They are a problem to keep as they only last a few days, but they can be frozen.

The artificial food on the market these days is very effective for aquarium fish. To diversify and balance their requirements, they can also be served some of the small, live prey that are commercially available - an important supply of vitamins and mineral salts - or fresh food or homemade mixtures.

One of the best fresh foods: mussels
This mollusk, prized by humans, is equally appreciated by fish, especially those marine species that refuse artificial food. Widely available, inexpensive, easy to freeze, mussels are a top-class dish: they are rich in proteins and carbohydrates, with few lipids.

Furthermore, they contain many minerals such as calcium, phosphorus, magnesium, iron and even vitamin C, which plays an important role in the fight against disease. Mussels must be well cooked before using and their shells and connective filaments removed. Cut them into pieces before distributing or freezing them.

Aquarium feeding with homemade food
An aquarist can easily make his or her own fish food - the possibilities are enormous. The following recipe is just one of many. Take some cooked white meat (chicken, turkey, ham) and some seafood (mussels, cockles), also cooked, and mince them with a little water to make a homogenous paste. A vitamin supplement, available from aquarium stores, can be added during this process.

This mixture can then be mixed with gelatin (from a sachet of powder, for example) so that the paste does not break up in the water, as this may become a source of pollution. One portion can be distributed immediately, while the rest can easily be frozen in ice trays. The models designed for small ice cubes are particularly recommended for making a large number of small portions. Once frozen, they can be slipped out of the ice trays and bagged up in freezer bags for use as required.

These were once very popular - so many aquarists have started with them! Nowadays they are out of fashion; in fact they are not the ideal food for fish and it is best not to use them. More complete and suitable products, particularly those in the form of flakes, are now available, even for goldfish.

Aquarium Feeding with Bloodworms
In fact, these are not worms but the aquatic larvae of various species of nonbiting mosquitoes. They are also commonly used as bait in fishing. Although it is sometimes thought that they live in mud, they are actually found in the water of areas that are well endowed with organic matter. It is useless trying to collect them yourself, but they are sold in the aquarium trade. They keep for a few days in the refrigerator, wrapped in moist newspaper. They are a good source of nutrition, as they are rich in protein.

Aquarium feeding with Tubifex worms
These are worms that are gathered from the mud in environments rich in organic matter. Some people think that they represent a risk when they are introduced into an aquarium, as they may carry undesirable bacteria. This risk is very slight, however, and there have been no reports of any serious incidents. The specimens which are commercially available can be kept for a few days in the refrigerator, in domestic water, but it is essential that this water is changed every day.

Aquarium feeding with Shrimps
Flowing and well oxygenated fresh water are home to small river shrimps of up 2 cm that swim along the banks. These are rarely sold live, but they can be found in a freeze-dried form. You will find that your fish will definitely enjoy them. The small marine shrimps found on some coasts are also highly prized, especially by marine fish. These are often sold in fishing stores along the coast. If you are lucky enough to live in such an area, you can also collect them yourself and keep them in a well oxygenated seawater aquarium.

Both types of shrimps can be frozen after being cooked and rinsed under the faucet, or they can also be ground and turned into a homemade dish.

Feeding with Small fish
Either fry or small fish can be used to feed marine fish doubtful about artificial food, or large freshwater fish. With this in mind, some aquarists breed prolific species that reproduce easily (Poeciliids, for example).

Other live prey (larger than 1 cm)
Keeping and breeding worms or insect larvae is also possible, though it sometimes proves time-consuming.
The range of live prey listed above is adequate for the needs of your fish.

Planktonic food
Freshwater or marine plankton contain a host of organisms barely visible to the naked eye (0.1-1 cm) but valuable for feeding fish, especially the fry. Collecting them in a natural setting is tiresome, and involves a risk of introducing potential microscopic hosts into the aquarium and spreading disease. Some planktonic food is available on the market in frozen form.

Brine shrimps
This is the "magic" food that every aquarist should breed. This primitive crustacean grows to 1 cm when adult and is called Artemia salina. They live in the heavily saline waters of salt marshes, feeding on the micro-algae that they gather with their filtering legs. Their characteristic most relevant to the aquarist is that their eggs can be stored dry, with their development interrupted, for use later on. Once the eggs are returned to salty water, they hatch rapidly and produce a larva called a nauplius, which measures only 0.3 mm and is particularly suitable for feeding to fry.
Feeding brine shrimps to fish is easy, even for an amateur (see box). They are still too big for the fry of some fish, however, especially marine species. In such cases, it is possible to use rotifers, animals halfway between worms and crustaceans, which have a crustacean-like shell and never grow to more than 0.2 mm.

These are more complicated to breed than brine shrimps, as they must be fed planktonic micro-algae which also need to be cultivated. It is therefore best to obtain them from a laboratory specializing in oceanography or marine biology, and to get advice on the essential steps required to look after them for a few days – the time within which the fry for which they are intended will be big enough and sufficiently sufficiently developed to move on to a diet of brine shrimp nauplii.

Aquarium Food Distribution
When they are healthy and well-adjusted to captivity, fish eat at a regular time and become accustomed to the spot where the food is given out. It is advisable to divide the daily ration into two parts - one in the morning and one in the evening, for example. The end of the day - 1 or 2 hours before turning the aquarium lighting off - is usually the most practical for the aquarist. In any event, food distribution offers a special opportunity to observe the behavior of your residents and check their state of health.

Allowing the fish to come and feed out of your hand is particularly enjoyable, but take care, because some large specimens have impressive teeth! If the water is too agitated, artificial or natural food may be dispersed too quickly and washed to a corner of the aquarium where the fish will not be able to recover it. Thereby creating a potential for pollution. The stirring of the water must therefore cease when food is being distributed and eaten.

Feeding the fry
When they are born, the fry of egg-laying fish feed on the reserves in their vitellin vesicle, as their mouth does not open until a few days later. They will then often accept the fine powders which are available commercially. If this is not the case. Give them brine shrimp nauplii for a few days – newly hatched nauplii are most suitable for the first two days. After that, they can be offered nauplii that are 48 hours old.

Daily production over a period of several days must therefore be planned for. If brine shrimps are unavailable, another option is the production of infusorians. Live-bearing fish (the Poeciliid family) accept artificial food from birth, and they also thrive on brine shrimp nauplii. As for marine fish, rotifers, discussed above, should be used, as they easily fit into the small mouths of the fry.

The dry eggs (known as cysts) are available in aquarium supply stores. They must be kept away from light and moisture. In order to make them hatch, salt water must be prepared with the following characteristics: temperature 25°C, salinity
35%, i.e. a specific gravity of 1.023. The salt water can be natural or reconstituted with special aquarium salts, or even with rough kitchen salt (easier for aquarists who do not keep marine fish).

The water can be colder and less salty (up to 20°C and 20%, i.e. a specific gravity of 1.014), but the hatching rate will be lower (50-60% against 80-90%). Any small glass or PVC container can be used - bottles, for example - although specialist equipment is available. The eggs are placed in the still water for a quarter of an hour, the time required for their rehydration. If we estimate that 250,000 eggs weigh around 1 g, a tiny amount (the tip of a knife, for example) will produce sufficient brine shrimps.

Aerate the water slightly to produce small bubbles, which will disperse the eggs, but be careful not to stir the water too vigorously, otherwise some of the eggs will crash against the sides of the container and will not hatch. The hatching occurs after 24 36 hours at 25°C, or after up to 48 hours at 20°C.

Finally, switch off the aeration: the empty shells will float to the surface, the unhatched eggs will fall to the bottom, and the brine shrimp nauplii will be swimming just under the surface. It is then easy to siphon them off (with an aeration pipe, for example) and strain them through a small filter (available commercially), or, alternatively, through the thin mesh of a piece of old curtain, or even a very fine pantyhose. To make this operation easier, you can group the nauplii together using a flashlight, as they are attracted by light. You can then go on to feed them to the fry. They will only survive for a few minutes outside salted water, and they will not eat on the first day after hatching.

If you want to keep them for several days to obtain larger or more mature larvae, special food is commercially available. This makes it possible to keep brine shrimps until they are adults this operation easier, you can group the nauplii together using a flashlight, as they are attracted by light. You can then go on to feed them to the fry. They will only survive for a few minutes outside

Adult brine shrimps are sold live in small sealed sachets containing salt water and air. They are passed through a sieve before being given to the fish, which enjoy hunting them down. They survive for a few minutes in unsalted water. Brine shrimps can also be bought frozen.

When feeding, it is important to avoid any overdosing, whether with artificial food or live prey, such as the adult brine shrimps pictured here.

These are microscopic, unicellular animals, easy to produce in fresh water. They are usually present in small numbers in an aquarium. Riccia, a surface plant, gives them a chance to grow, as they find food (organicmatter) on its leaves. They can also be produced by leaving a piece of potato, a lettuce leaf, or some paddy rice (unhusked rice, available in grain stores) to soak in a receptacle containing aquarium water.

What if a fish does not eat?
Sometimes a fish refuses to eat, or appears to be incapable of doing so. It is therefore a question of finding the cause and eliminating it. A newcomer to a tank rarely eats on the first day. This is normal, as it feels lost in its new environment. Small species and more lethargic fish are often dominated by their bigger and faster cohabitants at feeding time.

They must therefore always be fed separately, preferably with mobile, live prey, once the other fish have been distracted by other food. A fish can also refuse to eat if it is sick, and this will be reflected in its behavior, color, and other symptoms which may eventually be seen on its body. In this situation it must be isolated in another aquarium, treated, and given rich food. Comprised of live prey or fresh produce. Sometimes fish can systematically refuse to eat artificial food, although this is rare in fresh water, but less so in marine aquariums. There is no point in being stubborn: change to a varied diet based on live prey and homemade fare.

-       Give fish a varied diet;
-       Give them a little, but often. Two portions a day is ideal. For fry, the feeding can be more frequent.
-       Do not wait until the fish are sated and stop eating. Stop feeding once the ration is complete;
-       Siphon off any food surplus as quickly as possible, as the leftovers are pollutants.

A hard-boiled egg yolk is added to water in a glass. This forms microparticles which are then sieved.
This nutritious liquid is then given to the fry, taking care not to put too much into the tank, as it is always important to minimize pollution. Egg yolk, rich in proteinsm and lipids, can be used as a complement or as a replacement for other food.

Aquarium: Fish (Anatomy and Biology)

Aquarium fish Anatomy & Biology

There are more than 30,000 species of fish, more or less evenly distributed between fresh water and sea water, and of these some 1,500 are of interest to the aquarist. Fish embody a great anatomical and biological diversity and richness, and this can be clearly seen in aquariums.

Whatever type of aquarium you choose, a minimal knowledge of the anatomy and biology of the species you are raising is an essential prerequisite. The information below, presented in layman's language, allows you to keep your fish in good health, in the best possible conditions, to feed them appropriately so that they can grow, and to facilitate their reproduction — in short, to understand them better in order to take better care of them.

External Anatomy of a fish

The body
A fish is typically drawn as an elongated spindle, and in fact this is the most common form, as it makes it easier to swim in open water. These hydrodynamic characteristics permit rapid acceleration and not inconsiderable speeds (sometimes up to 20 km per hour) in a medium (water) that offers a certain degree of resistance.

However, there are other forms that are also all connected with the lifestyle of the fish in question: bottom- dwellers have a flat stomach, while those that live in water obstructed by plants and branches have compact, thin bodies that enable them to squeeze through the obstacles. This is equally the case with the countless fish in the coral reefs, which thread their way through the blocks of coral. Finally, there are certain fish that are unclassifiable, so varied and strange are the forms they flaunt, although they always correspond to a particular lifestyle.

The fins of a fish

Fish have several types of fins, each one playing a precise role. Their forms and names are often used to classify them into different families. Of the unpaired fins (i.e. consisting of a single fin), the most noteworthy are the dorsal and the anal fins. These serve to stabilize the fish when it is not going very fast or is coming to a halt, and they are tucked in when the fish swims more quickly.

The caudal fin (incorrectly referred to as the tail) supplies propulsion, in conjunction with the rear part of the body. In some species, particularly the Characins and the catfish, there is a small extra fin between the dorsal and the caudal fins, known as the adipose fin.

The paired fins, attached symmetrically to each side of the body, are called pectoral and pelvic fins. They are used for stabilizing, stopping, slowing down, or changing direction: vertically, from the water surface to the bed, and vice versa, from side to side, from left to right, from right to left. Fins consist of a membrane stretched on spokes, and they can all be tucked in along the body, with the exception of the caudal fin. The adipose fin is merely a fold of skin, without any spokes. When the spokes are longer than the fins they are known as spiny fins, and they can represent a danger to the aquarist, as in the case of the scorpion fish, for example.

The mucus, skin, and scales of a fish

Fishes' bodies are covered with a mucus that plays a double role: it reinforces the hydrodynamics by "smoothing" the skin, and it affords protection against the penetration of parasites or pathogenic elements. The latter point is extremely important, and it explains why fish must not be moved by hand: this risks damaging the mucus and facilitating the development of certain diseases.


Some aquarium fish have fins that are very different in shape or size from those that are found in nature. They are the result of patient breeding carried out by aquarists over a period of years.
The visual effect is guaranteed, but the fish's behavior is sometimes altered, especially its velocity when moving around.

Fish with large fins in the form of sails have little more than a remote relationship with their wild cousins, which have gone out of fashion and are no longer to be seen in tanks. The purpose of these selections can sometimes be in doubt: they undeniably result in highly attractive fish, but what advantage do they have over other stunning natural specimens?

Contrary to a widely held belief, the scales do not stick out of the body but are an integral part of the skin, and they are visible through a fine layer of transparent epidermis. When a scale is raised, damaged, or torn off, the skin itself is equally affected and becomes vulnerable to the action of pathogens.

Coloring of a fish

Every fish has a basic coloring that can be modified. Their shiny, metallic appearance, derived from the crystals present in the cells of the skin, varies according to the direction of the light striking them. A fish's color is a result of the different pigments located in the epidermis. These can change, slowly, for reproduction and camouflage, under the control of hormones, or more quickly, for flight or aggression, controlled by nerves. The coloring of a fish can also vary when it is suffering from disease or nutrient deficiency.


The coloring of a fish varies according to its age and mood. Some fish living in coral reefs
reject  individuals of their own species ora related species with a coloring similar to their own (Pomacanthids, also known as angelfishes, for example) because they consider newcomers as enemies wishing to appropriate their territory and their food supply. This is why their offspring have a very different coloring from that of adults, so as not to be considered intruders. In their desire to protect themselves, some fish adopt a camouflage to merge in with their surroundings, or, in contrast, reduce the intensity of their color to pass unnoticed.

Thus, the vertical black stripes on the scalare allow it to hide among submerged branches and plants (see drawing above). In some species, the male and female sport very different colorings, enabling them to be distinguished - a gift of nature much appreciated by aquarists!

This is true of a large number of the Cichlids in the African lakes. At mating time, the male can flaunt vivid colors, not only to seduce the female in the courting ritual but also to impress his rivals and scare them off. This occurs with the meeki, a Central American Cichlid - the underside of its head turns red at mating time.

The head of a fish

Whatever its form - conical, elongated, or stocky - the head houses some important organs: - first of all, there are the eyes, which have no eyelids and are highly mobile. This mobility, coupled with their position on the side of the head, allows a fish to command a broad field of vision - around 270°. In contrast, the clarity of its vision is unexceptional: beyond a certain distance, it distinguishes masses and forms rather than details.

Fish are very sensitive to variations in light - detecting low intensities of light, such as that of the moon - and they can recognize colors. - Next comes the mouth, with a size and shape related to its feeding habits. Carnivorous fish generally have a large mouth that can open wide and is endowed with an array of pointed teeth, which are sometimes curved towards the back to keep hold of their prey. Omnivorous and herbivorous fish have a smaller mouth, with flat teeth ideally suited to grinding food. The position of the mouth can similarly reveal eating habits:

• A mouth in the upper position indicates a top-feeder;
• A mouth in the terminal position is the sign of a fish that hunts underwater;
• A mouth in the lower position indicates a bottom-feeder.

How fish breathe

Water is aspirated through the fish's mouth, passes through the branchiae and is expelled due to the movements of the operculum, which covers them. There is always some water washing the branchiae of the fish. Oxygen requirements are not directly proportional to the size of the fish, with the smallest species being the greatest consumers of oxygen: ten fish weighing 1 g each consume more oxygen per gram of body weight than one fish of 10 g.

The barbels of a fish

Fish that live on the bed or in dark environments (colored or turbid water) have barbels around the mouth (Corfdoras, Botia, for example). These appendages have a tactile and sensory role. By complementing or replacing the eyes, they enable the fish to detect possible sources of nutrition.

The nostrils of a fish
Two or four in number, these are located in front of the eyes. They play no part in respiration but, extended inside the head by an olfactory sac, they perceive and analyze smells.

The operculum of a fish
This protects the branchiae and guarantees the circulation of water through the regular movements of
The valve, ensuring that the branchiae are always in contact with the water from which they extract oxygen.

The term "gills" sometimes incorrectly used, refers to the opening produced by the movements of the operculum, which serves as an exit for the water that has irrigated the branchiae.

The lateral line of a fish

Running symmetrically along each side of the fish's body, the lateral line is more or less visible, according to the species. It consists of a succession of pores that communicate with a canal situated under the skin. This important organ does not exist in any other vertebrates.

While the senses of taste and smell, highly developed in fish, allow them to recognize a greater number of smells than humans, at very low concentrations, the lateral line, with its special cells, detects and analyzes the vibrations of the water and sends this information to the brain. In this way a fish can be aware of the proximity of an enemy, of a prey... or of the approach of the aquarist (see box, p. 47). The importance of the lateral line is apparent in the blind tetra (Anoptichthys jordani), which never bumps into an obstacle even though it has no eyes.

Internal Anatomy of a fish

The sum of the internal organs accounts for roughly 50 to 60% of the body weight in a classically shaped fish.

The brain of a fish

This is fairly simple in fish, when compared to other more evolved animals. The parts corresponding to sight and smell are particularly well developed, demonstrating the importance of these two senses.

The fish skeleton

Obviously, this supports the fish's body, but it is less sturdy than that of a land animal, as a fish, partially freed from gravity, is "carried" by the water. Nevertheless, the relative fragility of the skeleton is a handicap and it is not uncommon to find fry that emerge from their egg "twisted".

The fish respiratory and circulatory system

This system is highly distinctive. The blood loaded with carbon dioxide is pumped by the heart to the branchiae, where it is oxygenated. Nature has provided fish with eight branchiae (four on each side), each made up of two leaves. The total surface area of these essential organs, when spread out, would be nearly equal to that of the fish's body.

The vivid red color of the branchiae is due to their abundant irrigation of blood; a darker color is a symptom of a respiratory problem. The branchiae are fragile organs, susceptible to damage from suspended sediment or parasites, resulting in a reduced intake of oxygen, with all its unfortunate consequences. After traveling through the branchiae, the vivid red blood, rich in oxygen, irrigates the body, and the oxygen goes on to break down the foodstuffs I the organs.

The fish in the Belontiid family, which live in water in which the oxygen is sometimes rarefied, have a special organ for trapping atmospheric oxygen.

The fish digestive system

This has no special characteristics, apart from the fact that the stomach can stretch to hold large prey, especially in carnivorous fish. This is where digestion starts, and it then continues in the intestine. With large prey the process can last several days, but in an aquarium, with artificial food, it will not take more than a few hours.

The fish excretory system

This allows undigested matter to be evacuated through the anus in the form of excrement, or feces. The urine is formed
in the kidneys, situated under the spinal column; it is evacuated through the urinary pore. It is worth mentioning that fish also excrete nitrogenous substances via the branchiae. All excreted substances contain nitrogen and are toxic for animals, but in a well-balanced aquarium they are eventually converted into nitrates and thus because no harm.

Countless aquarists have noticed that some of their fishes react more enthusiastically to their presence than to that of strangers. If they do "recognize" the person who looks after them, how do they do it? They are capable of distinguishing the special characteristics of the vibrations caused by the footsteps of such and such person, which are transmitted to the water of the aquarium. Not only has that, their sense of sight, although not perfect, helped them in this task of "recognition."

Which of the two recognizes the other first?

The swim bladder of a fish

Fish have a swim bladder, also known as an air bladder. This is an organ connected to the digestive system, which fills up with gas and helps fish to regulate their flotation when moving between two different types of water. They empty it to dive and fill it up when they need to come nearer the surface. Bottom-dwelling fish generally have asmaller swim bladder, or none at all, as they rarely swim in open water.

The reproductive organs of a fish

Males have two testicles that are linked to the vas deferens. While females have ovaries extended by the oviducts. In both cases the sexual products - the spermatozoa and ova - are expelled via the genital orifice. As the fertilization of the eggs is external and takes place in the water, there are no organs for coupling and fertilization, except in the case of livebearers.


When an aquarium is abruptly switched on in the morning, it is noticeable that its occupants do not immediately resume their normal activity. Some of them are on the bottom of the tank, some in the plants, and others remain almost immobile in the water. It is difficult to speak of sleep in the generally accepted sense of the word, but it is certain that fish have periods of restfulness, of varying degrees. This can be verified at night, with the aid of a small flashlight: the fish are practically stationary (apart from nocturnal species), but their eyes are not closed as they have no eyelids.


The sensation of pain is sent to the brain via the sensory nerves. As fish are endowed with the latter, it can be assumed that they feel pain when they are hit or wounded, and perhaps even when they are sick.


How do fish swim?

It is essentially the rear part of the body, particularly the caudal fin, which serves to propel the fish, while the other fins play a stabilizing and steering role. Of course, the more hydrodynamic a fish's form, the more it is capable of setting off abruptly and swimming quickly, indispensable for catching prey or fleeing an enemy. Aquarists are sometimes advised not to let quick and lively fish (like Barbs) cohabit with slower and more placid species (like loaches), as the latter may be frustrated in their attempts to eat the food provided by their owner.

General Fish Behavior in Aquarium

Fishes' behavior in an aquarium reflects their lifestyle in a natural habitat, albeit modified by the fact that they are living in a more cramped environment, coming into contact with other species more quickly and easily. Fish from the same species can behave differently from one aquarium to another, according to th capacity and the other occupants.

Fish Territorial behavior
When fish are in their original biotope, their territorial behavior is reproduced in captivity, and is sometimes even intensified. A territory is a living space – either permanent or temporary (as in the reproduction period) - with an extension proportional to the size of the fish. Its occupant rebuffs individuals from the same species, from related species, or even from totally different ones. The surface area must be sufficient for the fish to find refuge, foodstuffs, and fish of the opposite sex with which to reproduce. With some fish, particularly marine species, it is important to plan a territory in the aquarium that will provide shelters and hideaways.

Fish Group behavior
Strength is to be found in unity, and living in a group permits a better defense against enemies. Indeed, from a distance a group or school of fish takes on the surprising and intimidating an enemy.

Group life also facilitates reproduction, as an individual has a greater chance of finding a fish of the opposite sex. A group's unity and organization are governed by a series of signals which are invisible to human eyes: the use of the lateral line, for example, prevents fish from colliding with each other.

Fish Dominance behavior
The biggest members of a species dominate the smallest ones: when the latter get bigger they are ejected from the territory. Dominance behavior has practical and social implications, as the dominator will have priority in food and the choice of a fish of the opposite sex. At the bottom of the social ladder, the most dominated fish is permanently subject to aggression and harassment and has to hide most of the time, with its growth being prejudiced as a result. This is the case with some species of African Cichlids.

Prey-predator relationships of fish
Some fish feed on other smaller ones in a natural habitat, giving rise to incompatibilities in an aquarium: take care, for example, not to let South American Cichlids cohabit with Characins.

Sometimes an aquarium is a stage for aggression between different species. This aggression is always justified, as it is related to the defense of territory or offspring. It is a problem of space – these phenomena are rarely seen in big aquariums. However, a new fish introduced into a tank will often be considered as an intruder, or prey, and will be harassed.

Fish Growth and longevity

Unlike human beings, fish continue to grow throughout their life, quickly at first, and then more slowly with age. The size of fish in aquariums is mostly smaller than that found in the wild, undoubtedly as a result of the restricted living space at their disposal. This can easily be put to the test: an individual whose size has seemingly stabilized starts to grow if it is put into a bigger tank.

As regards longevity, this varies according to the species: a year, more or less, for the small species, and two to five years for the majority of fish. Some patriarchs live to the ripe old age of ten or more - these are large fish, particularly marine species. It is very difficult to postulate an optimal life span for a given species in captivity, as environmental conditions introduce too many variable factors.

In nature, the biggest fish often feed on smaller ones – obviously something to avoid in an aquarium.


Only put together those fish which are known to be compatible, particularly in the case of marine fish, and give them as much space as possible. Be sure to provide a number of nooks and crannies, appropriate for the dimensions of the residents. Another solution is to mix species with different lifestyles – for example, free swimmers (like Barbs and Danios) and bottom dwellers (like loaches) - that will not compete with each other.