Reverse osmosis, also known as hyperfiltration, is the finest filtration known. This process will allow the removal of particles as small as ions from a solution. Reverse osmosis is used to purify water and remove salts and other impurities. Using good quality RO water is essential for your reef or marine aquarium and Charterhouse Aquatics have a full range of two,three and four stage reverse osmosis units from all the biggest brands including TMC, Vertex and D-D.