How to make tap water safe for fish

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By Dr Peter Burgess, Senior Consultant to the AQUARIAN® Advisory Service (AAS) (part of Aquarian, and Mars Fish Care).

Water is for drinking[edit]

Tap water is supplied for human use and has to meet stringent drinking water standards. Indeed, it is generally of high quality and very safe... for humans that is. But certain chemicals in tap water, notably chlorine-based disinfectants, are potentially harmful to fish and should be removed before tap water is added to an aquarium.

Chlorine disinfectants[edit]

Chlorine-based disinfectants are added to most domestic water supplies in order to destroy any water-borne bacteria that may be present. The two common disinfectants are chlorine and chloramine, the latter being a more stable form of chlorine. Chloramine is now increasingly used in the UK.

Chlorine and chloramine damage to fish[edit]

Unfortunately, chlorine-based disinfectants are harmful to fish, damaging their delicate gill and skin surfaces. Fish that are exposed to high levels of these disinfectants will show signs of irritation, such as swimming erratically or attempting to leave the water. As little as 0.25 milligrams per litre of chlorine (that's less than the level typically present in tap water) is capable of killing fish in a fairly short period of time. Even the "hardy" goldfish is at risk.

At lower levels, these disinfectants won't necessarily be life-threatening to fish but can still harm them. Chlorine damage to the gills is particularly serious, causing the fish breathing difficulties. Affected fish may exhibit fast gill beats and gasp or "pipe" at the water surface in an attempt to get enough oxygen into their tissues. These symptoms can easily be mistaken for low oxygen problems, gill parasites or some other sort of gill disease, when in fact it could be raw tap water that's to blame. Tip: If you do suspect chlorine damage to your fish, increase aeration in the aquarium for a few days. This helps improve their chances of recovery.

Use of aeration to remove chlorine-based disinfectants[edit]

Some aquarists vigorously aerate tap water (e.g. overnight) to drive off the chlorine as gas. This technique does work, but be aware that even if you can no longer smell chlorine in the water it could still be present at levels that can harm fish. Aeration is far less effective in removing the more persistent chloramine disinfectant. So aeration has its drawbacks and isn't fool-proof.

Heavy metals[edit]

Either way, aeration won't deal with another group of potentially harmful water contaminants, namely the heavy metals. By "heavy metals" we mean elements such as cadmium, copper, lead and zinc. Fish seem to be much more sensitive to heavy metals than are humans.

Some water bodies are contaminated with significant amounts of heavy metals. Industrial mining is a major source of heavy metal contamination of water supplies. Being "pure" elements (rather than complex compounds that ultimately break down), heavy metals may continue to pollute watercourses for years or decades.

Apart from the water supply, there are other routes by which heavy metals can enter the aquarium. Zinc and copper, for example, may leach from galvanised or copper water pipes. Heavy metals may also be present in certain types of rock - something to bear in mind if you are thinking of collecting rocks from the countryside to furnish your tank.

Too much of a good thing[edit]

Tiny amounts of certain heavy metals are actually necessary for fish (and humans) to survive. Zinc, for example, is a component of certain enzymes (e.g. carbonic anhydrase) that perform key biochemical functions. Zinc is therefore an important dietary component for fish. But at very high levels zinc and other heavy metals may be harmful to fish.

Taking zinc as an example, fish are able to deal with excess zinc by actively excreting it from their tissues. But fish can cope only with so much zinc. Hence, if the zinc concentration in the water is very high it will become toxic to fish, causing gill damage and, in extreme cases, death. At sub-lethal levels zinc may inhibit the fish's growth and breeding potential.

Even tiny amounts of heavy metals may harm fish. It has been suggested that fish should not be exposed long-term to more than 30 micrograms per litre of lead or iron, or to more than 15 micrograms per litre of copper. (1 microgram per Litre is equivalent to 1 part in 1,000,000,000.)

Making tap water safe for fish[edit]

Evidently, chlorine-based disinfectants and excess heavy metals have no place in the healthy aquarium, be it a tropical system or a goldfish tank. Even when present at very low levels, these chemicals may affect our fish in various ways - causing breathing difficulties, poor growth, or poor breeding potential. These symptoms are not always glaringly obvious, or we may attribute our fish's poor performance to other causes. Bear in mind that fish cannot tell us when they feel under the weather! To remove chlorine, dechlorinator can be used.

Always use a water conditioner[edit]

Fish-keepers can make tap water safe for their fish by pre-treating it with a liquid water conditioner, obtainable from the aquarium store or pet store. Choose a product such as StressCoat (made by API) that instantly detoxifies both chlorine and chloramine as well as binding up any heavy metals.

Incidentally, StressCoat also contains a plant extract that is known for its tissue healing properties. This extract helps the repair of damaged fish skin. Unlike human skin which is "dead" on the outer surface, the skin of fish is a living tissue throughout, and is much more sensitive to cuts, infections and other damage. Causes of skin damage in aquarium fish include fin-nipping, fighting, accidental abrasion on aquarium décor, net-damage, and various skin parasites. If the natural barrier of the skin is breached or injured then the fish is more prone to invasion by bacteria, fungi and other harmful micro-organisms. So the sooner the fish's skin can heal, the better.

  • Reproduced by permission by Dr Peter Burgess of the AAS