It is not always necessary to use a deionising resin after an reverse osmosis unit as for most aquarists the water quality is well within the levels required directly out of the unit. Some people however want to remove all traces of salts from the water and by utilizing a deionising resin this takes water purification one stage further and will allow for total removal of nitrate and phosphate from the water as well as other salts. The deionising resin traps charged molecules and only allows uncharged molecules to pass through to produce very high quality water.