Over time your fish tank water will evaporate and need to be replaced.
If you live in a humid environment, or if you are amidst the humid months of summer, the rate the water evaporates and needs to be replaced will probably be slower. Lucky you.
During dry times of the year, or in dry climates, you’ll probably feel like you never get a break from replenishing the water in your aquarium.
Use Purified Water
You should always try to use purified water in your tank. As such, all of your water top-offs should be purified. A typical method of purification is through use of an RO/DI unit. Purified water can also be purchased. Local fish stores might also sell or provide aquarium water that has been purified or treated in some manner.
Matching Water Conditions
The water that you will be adding to the aquarium should be adjusted to similar temperature as your tank water before dumping it in. Dumping may also not be the best idea. It is better to trickle water into your tank at a slower controlled rate to prevent water change shock to your fish.
Top Off a Saltwater Aquarium with Fresh Water
Water that evaporates from a saltwater tank should not be replaced with salt water. The salt does not evaporate from the tank, just the water evaporates. As water evaporates from your tank the salinity level of the water will rise. Adding more saltwater will only make the salinity levels rise even more.
Water Level Fluctuation
It is best to not let the water levels fluctuate greatly in your tank as a result of evaporation for these reasons. Use freshwater to replace the evaporated water and you will keep the salinity levels consistent.
You may also want to condition the water with conditioners if you require.
If you have a salt water fresh tank, you will not need to add salt to the water that you use to top the tank off. It was water that evaporated, not salt.
Salt does need to be added over time if you lose salt for other reasons, such as salt creep. This is why you should frequently check and maintain the salinity levels of your tank.