Ditto maxweljames' and reefnut's comments. The evaporation rate is crazy. I add a cup or so in the morning before work with my b-ionic and a cup or so in the evening when I get home from work. Temp is pretty steady around 78-80 degrees with the fan on. These tanks are absolutely awesome to set up as nanos. Alot of work, but that's what makes it so enjoyable. You get a real sense of accomplishment when it finally works out. After all my work, it's smooth sailing now. I change 2 or so gallons each week with distilled water and compensate for evap with distilled as well. Sort of expensive to do this but when I used tap water, my cyano went out of control. Distilled totally cured that problem. And it's cheaper and easier than an RO system for such a small tank. My 125 gallon shark tank does just fine with tap water though. Maybe it's because of less intense lighting and a smaller percentage of water changed each time.