I go through about 1.5 gallons/day. It really seemed to go up a lot after I switched from the rinky-dink skimmer I was using on my old 55 gallon tank to the SWC skimmer. I don't know if that's the reason, but I'd estimate my evaporation loss about doubled when I made that skimmer switch. It kind of makes sense, I guess...much more water/air interface to allow evaporation.