You don't need to test the distilled water that will be your top off water. For water changes you do need to replace the water you take out with saltwater. Try to make it match the pH, salinity, and temp. as you do with your set up tank. Topping off the water can be a hassle b/c the water evaporates so quickly (at least for me). I have about a 110 g. system all together and I'd estimate 2 gallons more or less are evaporated each day. For you it might be a few cups a day, or since theres a cover maybe less. But it is essential to have the levels constant, so unlike FW
you can't just be lazy and not replace the evaporated when it gets really low. Water changes could be 10% a week. Or 5 gallons would work too (has great results and just need a 5 gallon jug). This is frequently replenishing the elements that the corals thrive on like calcium, alk
., magnesium, etc (in salt mix), and getting rid of dirty water and waste.