The salinity/specific gravity of sea water varies from one location to another. For instance, neither the Red Sea nor the Mediterranean is tidal and both have a high evaporation rate, therefore they are more saline than the open ocean. The Western Atlantic Ocean around the Caribbean also has a high salinity. The Dead Sea, of course, has the highest salinity of any sea in the world and no fish can live there at all. The salinity 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 fish in terms of basic biology 

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. 

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 or exchange gases like oxygen and carbon dioxide . TOPS ARE NOT RECOMMENDED.

NaCl ….Density
by.wt. g/ml

2.20 ….1.0139
2.30 ….1.0146
2.40 ….1.0153
2.50 ….1.0160
2.60 ….1.0168
2.70 ….1.0175
2.80 ….1.0182
2.90 ….1.0189
3.00 ….1.0196
3.10 ….1.0203
3.20 ….1.0211
3.30 ….1.0218
3.40 ….1.0225
3.50 ….1.0232
3.60 ….1.0239
3.70 ….1.0246
3.80 ….1.0254
3.90 ….1.0261

Specific Gravity and Salinity

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 1.000.  Sea water is denser and contains far more dissolved minerals, so that the Sp Gr is greater than 1.000. The concentration of total solids dissolved in a specified amount of water can also be expressed in terms of salinity i.e. gm/L. The table shows the relationship between specific gravity and salinity.

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.

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. 
2nd. Pick the closest specific gravity you want to have.  Note its % by weight. 
3rd.  Subtract the two percentages.
1.023 = 3.50% = 0.0350   1.024 = 3.60% = 0.0360   freshwater is 0% = 0
Multiply % difference x # of gallons of water X 8.344 = pounds of salt needed
Multiply % difference x # of gallons of water X 3785 = grams of salt needed
(0.035-0.00) x 5 x 3785 = 662.325

662 grams or 1.46 pounds of salt is needed to mix 5 gallons of fresh water to a specific gravity of 1.023
See Hydrometers and Refractometers  page for the correct way to read a laboratory hydrometer

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