RO water and RO salt water
Water from domestic taps is not really the best water to use in aquariums especially marine aquariums as it contains chlorine, phosphates, nitrate and heavy metals (lead, copper and many more) which are not ideal for fish keeping. For most coldwater and tropical freshwater fish, using a water conditioner is adequate for making the water safe for the fish. Discus and marine fish however require water of a higher standard to keep them happy.
To achieve the best results for your fish and invertebrates and also minimise nuisance algae, RO water should be used for both top-ups and water changes in both marine and freshwater aquariums.
RO is short for reverse osmosis which as the name implies, is the opposite of natural osmosis. To purify tap water, water is forced under pressure through a semi-permeable membrane which filters out minerals, contaminants and impurities making the water ideal for marine aquariums and also for drinking.
As RO water is pure water it has an unstable pH due to all the minerals being removed. If being used in freshwater aquariums, a mineral additive such as Tropic Marin Re-Mineral Tropic should be added to the RO water to stabilise the pH and to and achieve the correct hardness of water required for the aquarium.