Hi Stingray-
Actually, no- topping off with fresh H2O isn't the same as doing a water change. When water evaporates, it leaves all of the salts, proteins, mineral, poop, and goop behind. Topping off is important, because if you didn't the concentration of all of the stuff that is dissolved in your water will increase to higher, potentially toxic levels.
You should always top off with fresh H20 before you do a water change, too. If you don't, you may see eventual creep of your salinity levels to be higher than what you anticipate!
Norm
