Evaporation can lower oxygen levels in an aquarium. Fish swimming closer to the water’s edge than usual or gasping at the surface are sure signs that oxygen levels are too low. Adding new water is a simple way to help bring more oxygen into the tank.
If low oxygen levels are frequent (often in summer months), think about adding some extra aeration, such as an airstone and air pump, to increase oxygen levels.
If you have an open aquarium without a lid you could start using an evaporation tray to decrease the loss of water during the warmer months.