Water chemistry is a matter of life and death for aquarium fish. Most aquarium fish adapt poorly to water chemistry beyond their preferred range. Tap water tends to contain dissolved impurities, harmless to humans, that can cause serious problems for fish. Reverse osmosis, or RO, is a process that purifies tap water, making it ideal for a number of aquarium applications.
Reverse osmosis is a process of purifying water. RO uses semipermeable membranes and water pressure to remove impurities like heavy metals and dissolved minerals, producing soft, slightly acidic water. Such water can work as a sort of "blank slate" for water chemistry, free of chlorine, with no dissolved minerals. RO water has several important applications in both freshwater and marine aquariums.
RO units can cost a lot of money. Typical units may cost several hundred dollars. Additionally, their membranes wear out and need to be replaced. When selecting an RO unit, always compare prices of replacement membranes as well unit prices. RO has one other major downside: Since RO units depend on water pressure, they produce about five times as much waste water as they do purified water. This wastewater has all of the minerals that were removed from the purified water. While the wastewater is not good for aquarium fish, it is usually still fine for a lot of household applications, like watering plants.
Freshwater and RO
Many species of fish have narrow ranges of hardness and pH that they can survive in. RO water resembles the water many fish see in the wild. For example, much of the Amazon drainage is fed by rainwater, which closely resembles RO water. Fish from rain-forest streams, like South American cichlids, many tetras and some catfish, prefer RO water in their aquariums.
Marine and Hard-Water Fish
Even fish who come from hard water can benefit from RO water. For example, marine fish prefer water that's not just high in salt but high in other dissolved minerals. But RO water still makes a better starting point. High-end pet shops sell saltwater mix -- commercial preparations of salt, calcium and other minerals. These mixes work best with RO water. The dissolved minerals in regular dechlorinated tap water can throw off the levels of various minerals in the salt mix. Even freshwater fish who prefer hard water can benefit from starting with RO and adding minerals to it if the tap water has trace amounts of phosphates or ammonia.