Tropical aquariums usually contain a variety of different organisms and animals, each with their own tolerances to different water parameters. Many of the natural environments of these inhabitants are usually void of common tap water contaminants such as heavy metals, chlorine and chloramine (mixture of chlorine & ammonia). Many aquatic animals are thus extremely sensitive to these contaminants, so pure water is required to replace that has evaporated from the aquarium as well as during water changes.
A reverse osmosis system is the best option to ensure the best quality of water in the aquarium. Depending on the origin of the livestock, the water will sometimes need to be remineralised prior to adding it to the aquarium to ensure the desired levels of carbonate hardness (KH) and general hardness (GH) levels.