I'm not sure if you're asking this out of curiousity or to know how much water you should be adding back to the aquarium each day. If it's the latter, just make a mark on the aquarium with a permanent marker at the waterline. Then fill it back up to that line every day or two, and you should be fine. My 55g probably evaporates around 1/2 gallon every day when the top is off, and less than 1/4 gallon per day when I used to have a glass top covering about 80%.