Any body of water that is exposed to air is subject to evaporation to some extent, and aquariums are no exception. The water level drops over time - especially during the winter months when heated, dry air seems to almost suck the water right out of the tank as one watches - and new water needs to be added to keep the tank looking nice and filters and heaters functioning properly. Unless allowed to continue to the extreme, evaporation is more of a nuisance than a threat, but there are a couple of issues to consider.

Evaporation can be reduced by a good cover, trimmed as closely as possible to fit around the heater, filters and airlines. Likewise, wet/dry filters and even power filters should have reasonable covers to prevent too much contact between water and air. However, even if it were possible, it would not be wise to completely seal off the aquarium or filter unit: good gas exchange is essential to the health of an aquarium, and is the foundation upon which "biological filtration" is built.

As such, evaporation is unavoidable, and the periodic addition of make-up water is a necessity. Since only pure, clean H2O is evaporating, the perfect replacement water would be also very pure, such as Reverse Osmosis or even distilled water, but tap water (preferably de-chlorinated) can usually be used. Other than dechlorinators, it is not necessary or advisable to add salt or any other chemicals to the replacement water - these again do not evapo ...