Reef aquariums
Many reef aquarium keepers use reverse osmosis systems for their artificial mixture of seawater. Ordinary tap water can often contain excessive chlorine, chloramines, copper, nitrogen, phosphates, silicates, or many other chemicals detrimental to the sensitive organisms in a reef environment. Contaminants such as nitrogen compounds and phosphates can lead to excessive, and unwanted, algae growth. An effective combination of both reverse osmosis and
deionization (RO/DI) is the most popular among reef aquarium keepers, and is preferred above other water purification processes due to the low cost of ownership and minimal operating costs. Where chlorine and chloramines are found in the water, carbon filtration is needed before the membrane, as the common residential membrane used by reef keepers does not cope with these compounds.
I just copy and pasted this from wiki but this is why its used. If you just type in reverse osmosis to wiki you'll get more info. My dad hooked ours up so i dont know much about hooking it up but your tap water flows into the ro/di unit through a small tube and goes through all the filters and such and then comes out through another small tube with the water you'll use in the tank...you do have to add salt still of course