So now you may be wondering what to do in this matter. Well, you can top off the aquarium water all you like to keep the aquarium from emptying out, but I'd suggest to do a partial water change at this time by emptying out some of the current aquarium water, and then replace that removed water and top off the aquarium in the process. If you happen to notice any of your water parameters (nitrite, ammonia, pH, etc.) aren't where you'd like them to be after testing the water then it may require more additional water changes. Be sure the new water you add into your aquarium is the same temperature or within 3-5°F of the current aquarium waters temperature (or you may risk getting your fish sick with things like ich).
A few ways you can try to reduce aquarium water from evaporating quickly is by keeping a lid on the aquarium, although you still want some openings on your lid so that the aquarium gets proper air and water exchange so that the tank can cycle naturally without causing toxic CO2 buildup. Another thing you can do is try to lower the aquariums water temperature if you do have an aquarium heater, or try adding in an air diffuser to reduce heat. Also insure you don't have any leaks in your aquarium or filtration, and that any air pump tubes used have working check valves on them. If you do run an air diffuser it may make water gradually splash/bubble out of the aquarium if you don't have an aquarium lid.