There isn't much that you can do...evaporation happens. And you have to have the top open or screened to some degree for adequate ventilation. I live in Texas where it is usually dry. Just to see how much water evaporated, once I made a mark at the water line on the outside of the tank with a dry-erase marker. By the end of the week I was surprised with how much evaporated. With out the line, I didn't see the change as much through the course of the week. I usually replace the lost water when I do my weekly water changes.