The pH of water affects the solubility of metals, each metal with its own characteristic reaction. At very low pH levels, even aluminum can become free and toxic, though we never worry about the toxicity of aluminum, because it isn't soluble at normal aquarium pH values. Copper is another story altogether. Since copper is sometimes recommended for controlling invertebrates, the basic science of copper toxicity is important for a fishkeeper to understand. Acidic water can leach out concentrations of copper that are toxic to fish (though harmless to you) from copper pipes in your plumbing. Metals become more soluble in hot water. You may be more confident of the coils in your own hot water heater than I am of my landlord's. I've never had a water emergency so acute that I was ready to add to my tanks any water that had passed through the water heater. After any interruption in the water flow, whether in the public mains or your own lines, or after de-calcification of your water system, make sure that the very last of the household uses you return to, even after brewing coffee, is drawing some water for the aquarium.

The media used in chemical filtration, such as activated carbon, Purigen granules or PolyFilters, adsorb most heavy metals. Plants also act as chemical filters when they take up micronutrients like copper and iron in quantities far beyond what they can use, and store them in their tissues. Nutrient storage is a metabolic technique that algae aren't capable of. Duckweed performs especially well as a chemical filter, by taking up potentially toxic metals and storing them. I "export" these every time I net out duckweed. Sometimes I feed the excess to my barbs. I used to fret whether I might be poisoning my fish with these concentrations, until Diana Walstad relieved my anxiety on this point (in The Ecology of the Planted Aquarium, 1999); it appears that ingested metals quickly get bound to the organic molecules that fill the fishes' intestines. The unchelated copper, which is the toxic version, has to be absorbed by the fish directly from the water to be troublesome, usually across the gill membranes. So, the ingested chelated metals will soon pass out in fish feces. Nevertheless, they aren't permanently exported from the aquarium system until I discard the duckweed.

Mysterious brown staining. Sometimes aquarists who are using well water complain of brown stains on the glass and gravel and the decorations, stains which don't seem to be caused by diatoms, the usual offenders. In some cases these stains get deposited when freshly-drawn well water is added to the aquarium without giving it 24 hours curing time, to equalize its gas content with the atmosphere. If well water is depleted in oxygen, as it can be in certain aquifers, it often contains dissolved iron and manganese in ionic forms that oxidize as soon as they encounter oxygen. These hydroxides are insoluble, so the iron and manganese precipitate out. If your ceramic toilets and sinks tend to stain from your well water, you can leave these harmless precipitates behind in the water-curing bucket, if you'll just give your well water 24 hours to "cure." In a pinch, the process could be hastened by adding hydrogen peroxide (H₂O₂) to the water before adding it to the aquarium.

Phosphate. Water utilities may be adding phosphates in various forms to coat water mains and minimize scale and rusting and leaching of lead and some other some metals into drinking water. This too may affect the quality of water available for your tank. White phosphate-reducing granules that can be placed in your filtration are made of aluminum oxide. PolyFilters also adsorb PO₄. None of these media can be renewed. Phosphorus, in the form of phosphates, is an essential element for all life; its transformations figure among the aquarium's nutrient cycles. Phosphates also contribute to the alkalinity or buffering.

Fluoride. Fluoride added to tapwater by your municipal water utility, to strengthen children's teeth, won't have any effect on your aquarium fishes, not at levels commonly maintained.

Arsenic. In May 2000, permissible arsenic levels for U.S. drinking water were lowered from 50 ppb to 10 ppb, to bring them into line with standards set by the UN's World Health Organization. Dry and alkaline soils that are partly volcanic in origin are especially likely to contain arsenic; Albuquerque NM, for instance, has the highest arsenic levels in its drinking water of any major U.S. city. Water boards may be complying with additional filtration and additives, so you might want to find out what changes are afoot. More recently though, mining lobbyists and their senators have convinced the Cheney administration to roll back these guidelines for arsenic to their former levels, set during World War II, so this may be a moot point. But don't make the mistake of Roy Robinson, Albuquerque's water utility manager, and take arsenic for an essential electrolyte! "I feel arsenic is an essential metal— you can't live without it," the NY Times quoted him in April 2001. "It's like salt: too much of it will pickle you; too little of it will kill you." ("Had your arsenic today, Grandma?")