Also remember that the evaporation will be noticed in the sump, not the display tank. the water in your return pump section will go down. that's why its vital to keep an eye on it you don't want your return pump running dry. Is your tank drilled (does it have holes in the back of the tank where plumbing runs through to get the water to and from the sump) or is it a syphon overflow (where the plumbing runs up over the top of the tank and relys on a syphon to keep water flowing to the sump). Basically when you turn off your return pump that sump should be able to hold all water the drains from your display tank, then when you turn your pump back on and mark a line on the return chamber of your sump. This line is the water line that you want to top off to. Anytime its lower that that top it off back up to the line but never go over it or you can have a flood if you ever loose power.