Using Tap Water and Other Source Water for Aquariums

Tap water carries chlorine, chloramine, metals and sometimes nitrate. Learn how to condition and test it, the well-water and RO questions, and why standing won't remove chloramine.

Most aquariums are filled with tap water, but tap water is treated for human use, not for fish. Before it goes into a tank it usually needs conditioning and testing. The main issues are the disinfectant, dissolved metals, and parameters such as hardness, pH and nitrate that vary from one supply to another.

Chlorine and chloramine

Municipal water is disinfected with chlorine or chloramine to kill microorganisms. Both are toxic to fish and to the beneficial bacteria in your filter, so they must be removed. A dechlorinator (water conditioner) neutralises them quickly, typically in about 2 to 5 minutes.

Metals, nitrate and varying parameters

Water can pick up heavy metals such as copper and lead from plumbing; copper is especially harmful to shrimp and other invertebrates. Some municipal supplies also carry nitrate and phosphate, and hardness and pH vary widely by region and source. Check your water supplier's drinking-water quality report, which lists parameters from pH to chlorine, nitrate and phosphate, so you know what you are starting with.

Well water

Well and other untreated water often has no chlorine, so it may not need dechlorinating, but it can carry its own problems: high dissolved CO2 and low oxygen straight from the ground, iron, high hardness, and sometimes ammonia or nitrate. Have well water tested, particularly for heavy metals, and aerate it before use to off-gas excess CO2 and raise oxygen.

When to use RO/DI

If your tap water is unsuitable, or you keep soft-water fish and shrimp or a reef, reverse osmosis (RO) is the answer. An efficient RO unit removes 90% or more of contaminants, including nitrate, phosphate and heavy metals such as copper, leaving near-pure water that you then remineralise to the target hardness for your livestock.