Thursday, February 26, 2015

Aquarium : Sea water characteristics and information

The main difference between fresh water and sea water is that sea water contains a great many salts which give it certain specific characteristics, and these must be understood by any aquarist who wishes to keep marine fish.

Sea water Temperature

The temperature of tropical sea water varies little over the course of a day, or even a year. Furthermore, marine fish are generally more sensitive to abrupt changes than freshwater fish. The temperature in an aquarium must, therefore, be fairly stable, remaining at around 25-26°C.

Sea water Salinity

The most important salt found in sea water is sodium chloride (NaCl), widely used for domestic and culinary purposes, but there are plenty more. The salinity of water, i.e. the quantity of salts in the water, is expressed in 0/00 or in g/liter. The mean salinity of the Earth's oceans is around 350/00, or approximately 35 g salts/liter. Whatever its salinity, sea water boasts one remarkable property: the proportion of each element is constant.

Desalinated water does not therefore contain less of one or more salts, but the combination of salts is present in a lower concentration. The salinity of sea water varies according to longitude. It is at its highest in open seas in the tropics, it is lower near coasts and after heavy rain, and it is at its lowest near the poles (due to the influence of melting snow).

Sea water Density

In marine aquariums, it is not the salinity of water which is measured, but the density (often expressed as specific gravity, S.G.), which can be calculated according to the following formula:

There are no units of measurements. The saltier the water, the higher its density. The density also varies according to temperature (it goes down as the temperature goes up). The table overleaf shows the relationship between salinity and density with respect to temperature, which is relatively constant (25-26°C) in aquariums.

The density, expressed as specific gravity, a value which is easy to use, is all that is required to calculate salinity: it must range between 1.022 and 1.024.


Density is measured with a hydrometer, whose buoyancy increases as the water gets saltier. In the aquarium trade, most hydrometers also include a thermometer. The specific gravity at water level must be read with care; in fact, it is preferable to use the hydrometer outside the aquarium, as the movement of the water makes it difficult to read.

In this case, decant the water into a test tube or a transparent container (a PVC bottle, for example) and float the hydrometer in it. When it stops moving, read the value corresponding to the level of the water (1.023 in the diagram below, and not 1.022). To check whether your hydrometer is working properly, just measure the density of a distilled or very soft water: it must equal 1.000. THE CH

Unlike the general hardness (GH) which is used to describe fresh water, carbonate hardness (CH) is used in sea water, where it serves to measure the quantity of calcium and magnesium carbonates and bicarbonates present. This is crucial for maintaining the pH, and for growing corals, which have skeletons made of calcium carbonate.


As sea water is salty, its pH is therefore higher than that of fresh water. Pure sea water in the middle of the ocean has a pH of 8.3- Near the coasts, this drops to about 8 or a little less, as its dilution with fresh water lowers the salt content. The pH of sea water in an aquarium must vary between 8 and 8.5; beyond these values, animals will experience certain physiological problems.

Variations in pH in a marine aquarium Sea water contains a great deal of calcium carbonate and bicarbonate, and there are only slight variations in pH in a natural setting. It is a different matter in an aquarium, a restricted habitat operating as a closed cycle. The pH must not fall below 8, but a slow and regular decrease in this parameter may be seen.

Why? The water in an aquarium sometimes contains too much carbon dioxide, which has a tendency to lower the pH. What can you do? The first step is to measure the CH: - if it is under 7.2°CH, add calcium or replace some of the water. This situation is, however, fairly rare in an aquarium without corals, solely occupied by fish; - if it is over 7.2°CH, there is an excess of carbon dioxide. Stirring of the water must therefore be increased by using diffusers or an electric pump.


This occurs in the same way in sea water and fresh water. In a marine aquarium the vegetation is often less abundant than in fresh water, and so the nitrates, the end products of the nitrogen cycle, will have a tendency to accumulate. At high doses these pose little danger to fish but are toxic for invertebrates, especially corals. It is therefore important to eliminate them by partial, but regular, water changes.


The European Union has set compulsory standards for drinking and environmental quality, but the strict legislation in the UK goes well beyond these. In England and Wales, for example, domestic water is monitored by the Drinking Water Inspectorate, which regularly checks up on the practices of the water companies and investigates any possible infringement of the law.

The Environment Agency, on the other hand, is responsible for the quality of water in rivers, estuaries, and coastal areas. It issues licenses to discharge waste into these waters and takes chemical and biological samples to monitor the effect on the environment. The results of these controls are available to the public.

Once a marine tank has been put into operation, the nitrogen cycle is slower to take effect than in a freshwater tank: around 3-4 weeks (although this is a generalization, as every aquarium is unique). Fish or other animals must not therefore be put into the water during this period, although the length of time can be reduced by various means, based on the principle of introducing bacteria.

In any event, measuring the nitrite levels is an excellent indicator of the progress of the nitrogen cycle. Once the water has been put into the tank, this parameter must be measured regularly; when the quantity of nitrites goes down close to zero, the nitrates appear and you only need to wait a few days before inserting the fish. Nevertheless, measuring the nitrites at regular intervals is still highly recommended, as long as the aquarium is in use.


Sea water contains more than 60 elements, some of them in microscopic amounts: for example, there is 1 g/m3 of gold in sea water. All the solids dissolved in sea water serve a purpose, and that is why the salts that are used to reconstitute water must be of excellent quality.

Some substances can accumulate in sea water and in high concentrations give rise to concern. This is especially true in the case of organic matter, but it is possible to eliminate them by partially changing the water or using certain devices, such as an aerator marshes. Furthermore, good sea water cannot be reconstituted using poor quality fresh water.

Where and when to collect natural sea water? The ideal solution would be to go to the open sea, where the water is likely to be less polluted and to have more constant characteristics. Near the coasts, the following must be avoided: urbanized or industrialized areas and ports, which are susceptible to pollution; anywhere near river mouths, estuaries, or bays, where the water is desalted; and areas of stagnant sea water (pools at low tide) and salt marshes.

sea water characteristics collection of water from coastal
Coasts with sand dunes are suitable in principle, but the water is often laden with suspended sediment. Rocky coasts are preferable regions from where water can be collected. The best periods for collection are autumn and winter, because plankton develop in spring and tourism increases the risk of pollution in summer. Calm weather is preferable, in order to avoid suspended material, although a heavy swell reoxygenates the water.

In this case, the water can be collected 1-3 days later, the time in which the suspended material turns into sediment. However, the water must be filtered in all cases, first roughly and then more finely.

The reconstitution of artificial sea water The quality of the fresh water used is important: it must be as pure as possible. It is best to use water with a hardness of less than 8.4, although reconstitution is still possible with higher levels, providing the CH is equal to at least 75-80% of the general hardness value.

Take care to avoid water containing nitrates (often found in farming areas), to which invertebrates are very sensitive, or metals, toxic for some animals where present above certain limits. Making sea water in an aquarium, before putting it into operation Fill the aquarium with fresh water and aerate it for 24 hours. Calculate and weigh the quantity of salts to be dissolved, then introduce them into the aquarium. Then just aerate for another 24-48 hours and check the density, adjusting it as required.


Several companies have special aquarium salts on the market, and it is even possible to find concentrated sea water. Some salts are intended for marine tanks for fishes, others for aquariums with invertebrates.

Their quality is satisfactory, although there are likely to be improvements in the future, and, as they are enriched with calcium, micronutrients, and vitamins, they are obviously relatively expensive. There have been no adverse reports to date about the use of these salts in aquariums: in those areas where accidents do occur, they are usually due to miscalculations on the part of the aquarist.

Making sea water for storage and back-up 

The method is the same, except that plastic food containers are generally used. The quantity of salt can be multiplied by three or four to manufacture concentrated water that will therefore occupy less storage space.

Adjusting the density 

• The density is too high Part of the water is siphoned off - this can be stored for later use - and the softest water available is added, taking care to measure the density. When the water level of a marine aquarium goes down because of evaporation, it is not the sea water which is evaporating but the fresh water, and it is therefore the latter which must be added to make up the level.

The addition of sea water would entail an increase in density.

 • The density is too low In this case, salts must be added. These must be dissolved beforehand in a container which is then gradually emptied into the aquarium, with constant checks on the density. Both these operations must be performed with care if the aquarium already contains fish, in order to avoid causing any excessively abrupt changes that could be detrimental to the fish.

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