Maintaining and correcting the specific gravity of an aquarium
Maintaining the specific gravity in aquarium water is fairly simple and straightforward. All evaporated water should be replaced with freshwater (or an equivalent such as Kalkwasser) and should be added daily to reduce swings in salinity/specific gravity. Small variations in specific gravity are not problematic and are natural occurrences on reefs. No more than plus or minus 0.001 in one day is recommended. Adding 2L of fresh water to a 200L tank will only vary the specific gravity by 1% (around 0.00025). Small amounts of saline water removed by the skimmer or splashing can be corrected with water changes.
If the specific gravity of the aquarium is wrong, it should be changed gradually. If it is too low, it can be gradually raised by replacing evaporated water with seawater until the correct specific gravity is achieved. If the specific gravity is too high, small changes can be made simply by doing water changes with water at slightly lower than the correct specific gravity.
For larger changes, tank water can be removed and replaced with freshwater. Changes of no more than 0.001 to the specific gravity per day should be made. For example, to reduce the specific gravity from 1.030 to 1.029, 3% of the volume of tank water should be removed and replaced with fresh water. In a 200L tank this would equate to 6L. The fresh water should be added gradually to sump to ensure it is well mixed before returning to the tank.
Specific Gravity: Oh How Complicated! by Randy Holmes-Farley.
...salinity values that are "optimal" for keeping marine aquaria. That has been addressed in previous articles, such as this article by Ron Shimek.