You can top it off, as long as you are still keeping to your regular water change schedule. In the winter I loose nearly 1/2 a gallon daily in my 65 due to evaporation. I top it off every couple of days, I keep gallons of dechlored water next to the tank. I've let it go down between weekly water changes, the hob gets noisy, and you can start to see the surface of the water beneath the frame.
Tighter lids are the trick, I've tried seeing how much difference 2 10 gallon tanks evaporate, 1 with no top, 1 with a good tight fitting lid. The one with no top looses more than twice as much water to evaporation in a week.
Tolak
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