Form of purification, often abbreviated to RO, that removes dissolved salts and other impurities from water by applying pressure to it on one side of a semipermeable membrane. Pure water molecules are able to pass through the membrane while the majority of others are not, with more-or-less pure water the result. Sometimes used in combination with deionisation to remove additional mineral ions. Pure water can be harmful to fishes and is normally remineralised to some degree using proprietary products.