Specific gravity and salinity
A freshwater fish is surrounded by water which is less dense than its body fluids. Due to a phenomenon known as osmosis, water is absorbed into the body and a fish must excrete water constantly so that it does not burst. The marine fish faces the opposite problem: it is constantly losing water to its surroundings so that it must drink copious amounts of water and excrete only salts.
Variations in S. G. will occur as a result of the evaporation. Only pure water is lost during this process and so evaporation losses should be replaced with fresh de-ionized water and not a prepared saltwater.
Losses caused by evaporation can be automatically replaced using self-acting top - off devices. A close - fitting cover will cut down evaporation losses as well as conserve heat in the aquarium. Therefore your tank cannot cool itself. TOPS ARE NOT RECOMMENDED.
How to calculate the amount of salt needed to change your specific gravity.
1st. Pick the closet specific gravity from the chart at the left you have now. Note the % by weight.
Multiply % difference x # of gallons of water X 8.344 =
Multiply % difference x # of gallons of water X 3785 =
450 grams of salt is needed to mix 5 gallons of fresh water to a specific gravity of 1.023
Specific gravity is simply the ratio of the density of any liquid compared to the density of distilled water (which has a specific gravity of 1). Sea water is denser and contains far more dissolved minerals, so that the S.G. is greater than 1.
(The concentration of total solids dissolved in a specified amount of water can also be expressed in terms of salinity i.e. gms/litre. The table shows the relationship between specific gravity and salinity.)
The salinity/specific gravity of the water has an effect on the fishes living in it, and here we come to one of the fundamental differences between saltwater and freshwater fishes in terms of basic biology