Maintaining Proper Salinity Levels
Initial water calibration
When the tank is setup, the salinity of the tank should be measured with a hydrometer such that the water has a specific gravity between 1.020 - 1.025. Personally, I like 1.023 because it's easy to remember like 1,2,3.
If it is too high, then remove some water, and add de-ionized water, and then wait for about an hour to equilibrate, and then measure again. Just remember to go slowly.
If it is too low, then just add salt water. I recommend using just premixed 1.023 specific gravity sea water, but if the measurement is really far off then a stronger mix of saltwater may be necessary. Once again, go slowly, and give the water time to equilibrate.
Once the salinity is at a stable level, measure the water line in the sump. This should be roughly at 50% of the sump's total volume. Just remember to leave enough water space in the sump for excess water if the power should go out. Either make a mental note of the water line, or better yet, use a marker and mark the fill line as "MAX". This maximum fill line will be used to maintain the salinity of the tank. From now on, it is safe to assume that the MAX Line represents "Natural Salinity of Sea Water".
...the MAX Line represents the "Natural Salinity of Sea Water"
Periodically, the salinity at this line should be measured in order to verify that this line actually does represent the proper salinity. If it doesn't then adjust the salinity using the method above. For large tanks this shouldn't be a common occurance.
Water will evaporate from the tank throughout the day, and even more so with the lights on. In order to maintain the salinity, the water level needs to stay at the MAX line mentioned above. Fill the sump with de-ionized water so that the water level stays at the MAX line. Do not use salt water for top off, otherwise the salinity of the water will go up!
When salt water evaporates, the water portion goes away, but the salt remains. As a result, the salt water becomes more concentrated, and it needs to be diluted by topping off with de-ionized water.
Never top off with salt water!
No more than fifteen to twenty percent of the water in a tank should be changed on a regular basis for a healthy tank. Replacing too much water an any given time can upset the pH and temperature of the tank.
Before performing any water changes, make sure that the water line in the sump is at the MAX line mentioned above. Next, count the number of gallons of clean sea water that you will have for the water change. Knowing the number of gallons of clean sea water, remove exactly the same amount of water from the sump. The removal of water from the sump should be completely done before the addition of any water.
For example, if you have ten gallons of clean sea water, then remove ten gallons of water from the sump, and then add the ten gallons of clean sea water. Do not remove five gallons, then add five, then remove five, and add five. Doing so will not affect the salinity of the tank; however, it will reduce the effectiveness of the water change.
Replacing too much water at one time can upset the chemistry of the tank.