Tank water evaporation is a normal part of the cycle in a fish tank. However, it can cause problems if there's too much evaporation too quickly. As the water evaporates, the amount of water for your fish decreases, which can make the water quality also decrease as more fish waste gets added to less water. Preventing evaporation will help keep your fish healthy.
Turn down your tank heater to maintain the water temperature at the lower end of the appropriate temperature range for your fish. The warmer the water, the more it will evaporate. For example, freshwater angelfish need water temperatures in the range of 75 to 82 degrees Fahrenheit. Keep the temperature closer to 75 degrees to prevent evaporation.
Use a cover for your fish tank that encloses the top completely. This way, any evaporated water will condense on the cover and fall back into the tank.
Turn the light off at night. The tank light also heats the water and increases evaporation rates.