Aquariums evaporate water, there’s not way around it. Whether the tank is reef ready, covered with glass tops, or whatever, the tank will lose some water over time. But some designs lose water at a greater rate than others. Open top aquariums or tanks with high intensity lighting will lose water at a rapid rate. Also affecting these rates is the footprint of the aquarium. If the tank is tall and narrow, it has a small footprint and a smaller surface area for exposed water to evaporate from. The shallow, large footprint aquariums will have the greatest evaporative rates, with some tanks losing 3-4 gallons per day in some cases. To prevent evaporation, cut down on the high intensity lighting and decrease water surface agitation, among other options of course.