RO (Reverse Osmosis) water has been used for a long time in Marine Fishkeeping, but in recent times has also become increasingly popular among fresh water tropical fish enthusiasts. This article addresses some of the basic questions about the use of reverse osmosis water. It is not an in depth study, but intended only as a guide to the simpler aspects so further study and research will be needed for specific applications.

Nitrate levels in aquariums need to be controlled as at high levels they compromise the health and well-being of the fish. This can eventually lead to bacterial and fungal infections, Whitespot, Fin-rot and other diseases that take advantage of compromised immunity. High nitrate levels are often the cause of excessive algae growth too.

Ammonia is formed from the metabolism of protein and is the major waste product of fish. The majority of ammonia from fish is excreted through the gills, with relatively little being lost through urine and feces. Ammonia is also formed as uneaten feed or other organic matter in an aquarium decomposes. High concentrations of ammonia in the water make it difficult for fish to eliminate ammonia from their bodies. This buildup of ammonia can cause stress, gill and internal organ damage, and eventually death.

Nitrite is formed by the conversion of ammonia by nitrifying bacteria. Nitrite is toxic to fish because it binds with the hemoglobin in fish's blood to form methemoglobin. Hemoglobin carries oxygen through the body while methemoglobin does not, so fish in high nitrite waters may suffocate even if sufficient oxygen is present. If enough methemoglobin is present in a fish's blood, it will cause the blood to appear brown instead of red. Consequently, nitrite toxicity is often called "brown blood disease."