Reverse osmosis, or R.O. water for short, is water that has been stripped of most of its mineral content by a membrane that only allows certain molecules to pass through it's matrix. The result is puriﬁed water that has very low hardness and pollutant levels and can be used for keeping demanding ﬁsh or marine invertebrates such as Discus or Corals. It's a great way to keep soft water ﬁsh or a planted aquarium in a hard water area, or freeing yourself from the constraints of local water hardness.
Although R.O. water is ideal for expert aquarists keeping delicate species, it is equally suitable for the average community tank - especially in areas where tap water contains high levels of nitrate or phosphate.
As R.O. water is so low in essential minerals, it is normally necessary to add buffering products to prevent problems with pH. Different formulations of these essential mineral salts are available for freshwater or marine use and should be added before using the water in your aquarium. Using this water without restoring buffering capacity can result in harmful swings in pH - always measure and adjust this water according to your needs, or ask a member of staff to add a measure of buffering agent to your barrel of water. Although this puriﬁcation process removes a large percentage of the dissolved mineral content, the quality of the source water will have some bearing on the end product and as a consequence, some compounds can still be present at very low levels. If you require water of greater purity, further processing through a deionising resin unit can provide water with the lowest achievable levels of mineral content. Ask a member of staff for more details. When dealing with puriﬁed water, it's important to remember that storage and transport containers can affect any measurements of dissolved solids, especially if any barrels used have previously held salt, substrate or even hard water. pH is inﬂuenced by oxygen and carbon dioxide content, so these can vary dramatically in water with very low mineral content and give little indication of purity.