 Aquarium Reverse Osmosis is the mechanical process to remove all impurities of tap water for aquarium use. Aquarium Reverse Osmosis works on the principle of squeezing water through a tiny perforated disk with holes which are only big enough for the water to pass through.
These units often come with multi stages where the water gets treated before its goes to the membrane. You often get a sediment filter and a carbon filter as pre stages. The sediment filter catches particles in the water which are large than 5 micron so that there is less chance of the membrane becoming blocked. The other stage is a carbon filter which neutralises many harmful chemicals in the water such as chlorine and chloramine which is present in all UK drinking water.
There are many aquarium reverse osmosis units to choose from but make sure you can get replacement parts particularly the replacment cartridges as they need to be changed at least every 6 months. Also you need a good stock mains pressure to operate these units as the water needs enough pressure to push throught the actual reverse osmosis membrane. | 
