In the wild, fishes occupy a variety of niches, from high-acid jungle pools to extremely hard lake waters, from still sloughs filled with rotting vegetation to clear rushing mountain streams. In aquaria, fish are often forced to endure whatever water conditions come from the local tap.
A number of substances that are found in quantity in water from the tap are found, if at all, in only minute quantities in the wild. Many of these are highly toxic to fish and plants, including heavy metals such as cobalt, nickel, mercury, chromium, copper, zinc and lead.
Adding to this concern in some areas is the use of copper and lead pipes. Although some of the danger is reduced in older systems due to chalk deposits, in newer systems more than 2 mg/l of copper may be dissolved in water that has stood in the pipes for sevearl hours. Quantities of 0.5 mg/l of copper have proven fatal to many aquarium fisih. Hot water piping and copper boilers are of particular concern, since copper solubility is dependent on heat. Also, the softer the water, the greater the toxicity. Not only the fish are affected by high copper levels; the entire aquatic eco-system is harmed.
Should you be concerned about the possible toxicity of your water, tests are available to measure the level of various metals, and products have been developed to neutralize their effect in the aquarium.
— G.C.K.A. Newsletter, Sept/Oct. 1998