in all aquariums your gonna have some water evaporation so with a salt water aquarium do you need to replace the evaporated water with saltwater or can you add fresh water as long as you keep your salinity in check?
If you have a sump, draw a line (or put tape) in the return compartment, and if it goes below that line, fill it up. If you don't have a sump, then somewhere inconspicuous, make a li'l mark on your dt to tell you if how much you need to top off.
My sump has a diy'd auto top off -- when the float valve goes low, it refills it back up. Since installing it last month, I've never had to do nightly top offs. I just have to make sure my RODI reservoir is full...hehehe