I don't know how hot it gets in your house, but I find I need to cool my tank off via evaporation. Well also the light heats up the tank...
With the water condensing on the glass top would it leave mineral deposits? That would be a pain to clean all the time, as it would also interfere with light (although I guess that would only be a problem with plants...).
I sometimes use distilled water, or don't top off at all (the water drops down about an inch or so). If you're using amquel and the bottle says 5ml for 10 gallons, then it would be .5ml per gallon. Though I don't know how many water treatment plants actually treat w/3.2ppm chloramine. Here I think it may be less than half that amount. I use about 4-5 drops (that's like .15ml or something at a rough guestimate).
Perhaps people don't need to treat top off water b/c there's excess amquel or dechlorinator in their tank to take care of the top off water?
Edit: For some reason I egocentrically assume that everyone has chloramine in their water. Although most water treatment plants seem to be switching over, so I guess many do. I think it has something to do with aging water systems and pipe corrosion, as well as stability of the dechlorinator. I don't think many of those brita type filters handle chloramine. What do they do? Split off the chlorine and leave the ammonia, or vice-versa? I wonder how many home filtration systems handle chloramine... So are we drinking ammonia now?? Note: this is totally theoretical, I guess I have some research to do... Sorry for the digression....