Using tap water & topping up
Evaporation and fluctuating water levels are extremely common in aquariums, especially in summer or in very warm homes. Although it may not look like evaporation causes problems, it’s important to top up the water levels regularly.
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 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.