Confused about testing salinity in clean water?

[SCIFI_3D_zoo]

I thought I read you saying to test salinity of your RO/DI water? Making sure this, and pH, temp. are the same as your tank. I know you can't test pH of filtered water. I'm trying to figure out how to deal with temperature b/c my setup is in the garage in Florida. But salinity?

Even if I misread this or it's a typo let me see if I understand this right. The salinity in your tank is a set amount of salt and the only way that goes is through stuff like evaporation. It has nothing to do with the amount of water in your tank. So you only test the salinity of your tank... and if you need more salt you mix it in your filtered water and add that to your tank. Otherwise you don't check salinity or add salt to your filtered water?

[melev]

What I'm recommending in my article is measuring the new (aged at least 24 hours) saltwater to make sure it matches the display tank's pH, temperature, and salinity before performing a water change.

Top-off water doesn't need to be tested because only small amounts are added each day, either manually or by an automatic replacement system. That water should just be RO/DI with nothing added. As water evaporated from the display tank, salinity rises slightly. The RO/DI water being added to the tank will keep salinity stable and relatively unchanged.

The way salinity can change significantly is if:

1) The skimmer pulls out a lot of skimmate.
2) The tank is not top-off for a significant amount of time.
3) The system has been top-off too much by adding more RO/DI water non-stop accidentally.

If #1, it may be necessary to add some salt to the top off water to replace what the skimmer is removing. This is usually unnecessary unless the skimmer is pulling out a couple of gallons of skimmate daily.

If #2, top off twice a day or at least once a day. For smaller tanks, it may be necessary to top off often to avoid salinity swings. Usually once a day works for most tanks. Myself, I prefer an automatic top-off system to add it throughout the day as the float switch drops. More water is added until the float switch rises.

If #3, please read this article to avoid a fatal conclusion:
http://reefkeeping.com/issues/2007-05/ml/index.php