Water is for drinking.. .'Our water is supplied for human use, not for keeping fish!" replied a Water Company representative when approached by an irate aquarist who was blaming the local tap water for his fish deaths.

Of course, the Company employee was quite right: tap water is primarily for human use and has to meet stringent drinking quality standards.
Indeed, it is very safe for humans. But fish and humans react differently to the chemicals in tap water.

We merely drink or bathe in water, whereas fish live and breathe in it. Additives, Hardness and pH aside, it generally isn't the water source itself that can cause fish keeping problems (although heavy metals may pose risks). Aquatically speaking, our major concern lies with the disinfectants that the water companies add to tap water

Avoiding the trots

Chlorine compounds are great for killing water-borne bacteria that could otherwise give us humans a bad stomach upset Unfortunately for us aquarists, chlorine is highly toxic to fish, harming their delicate gill and skin surfaces Water companies add sufficient chlorine to the water supplies to achieve a concentration of 0.5 to 1.O milligram of chlorine per litre at the tap.
Enough to kill those gut-thumplng bacteria . . and our fish.

Avoidance reaction

Fish that are exposed to high levels of chlorine will show an avoidance reaction and may attempt to leave the water. Trout and other salmonids are particularly sensitive to chlorine, but high chlorine levels can irritate and harm just about any fish species, even the so-called "hardy" Goldfish.

As little as 0.25 mg/L of chlorine (about half the amount in tapwater) is sufficient to kill most fish fairly quickly. But more insidious are the sub-lethal chlorine levels that we may inadvertently expose our fish to.

It has been suggested that fish should not be exposed to more than 0003 mg/L of chlorine This "safe" level is less than 1 per cent of that found in tap water.
To look at it another way, over 99% of the chlorine in tapwater needs to be removed in order to render the water safe to fish!

Chlorine damage

Chlorine reacts with the fish's gills and skin Gill damage results causing the fish breathing difficulties. Affected fish may develop fast gill beats and may gasp or "pipe" at the water surface These symptoms can easily be mistaken for low oxygen problems, gill flukes or some other sort of gill infection, when in fact it could be the raw tap water that's to blame.
(Tip: If you do suspect chlorine/chloramine damage to your fish, increase aeration for a few days. This may improve their chances of recovery.)

Inactivation by organics

Chlorine is inactivated by organic matter, so adding untreated tap water to a mature aquarium is less risky (well, slightly less!) than adding it to a recently set up system. Those of you who show your fish should be extra cautious about chlorine toxicity as there will be little organic matter in the show tank to mop up free chlorine. So beware of topping up a show tank with raw tap water always treat the water first, even if it's just a small quantity.

As indicated earlier, exposure to even tiny amounts of chlorine can irritate the fish, and could conceivably mean the difference between the fish getting 1st in a Class and being unplaced.

Total chlorine eradication is therefore our goal, whether we show fish or not.


This is a more stable form of chlorine. It has the advantage over chlorine in that it doesn't react with humic acids and other organic acids that are present in some water supplies. These chlorine-organic acid reactions can yield compounds known as trihalomethanes that are potentially harmful to humans. Hence chloramine is nowadays added to some tap water supplies to safeguard human health. Good news for us, but not for our fish and that's because chloramine isn't easily removed from tap water.

Removal of chlorine-based disinfectants

Some aquarists vigorously aerate tap water (eg overnight) to drive off chlorine as gas. This technique works, but be aware that even if you can no longer smell chlorine it could still be present at levels that can harm fish. Aeration is far less effective in removing the more persistent chloramine. If you are 100 per cent sure that your water supply company NEVER uses chloramine then aeration is a satisfactory method of dechlorination. If not, then aeration alone is risky in my view. Either way, aeration won't deal with another group of potentially harmful contaminants, namely heavy metals.

Heavy metals

By "heavy metals" we mean elements such as cadmium, copper, lead and zinc. Fish appear to be much more sensitive to heavy metals than are humans. Hence, water that is deemed safe for us may contain levels of heavy metals that could harm fish. 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 even after local mining has stopped.

Heavy metals also accumulate in the sediments on the lake and river bed, reaching concentrations that are far higher than in the water itself.

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.
It is therefore a good idea to run the tap for a few minutes before drawing off water for aquarium use Heavy metals may also be present as seams within certain types of rock something to bear in mind if you decide to collect rocks from the countryside to furnish your tank.

Too much of a good thing..

Of course, tiny amounts of certain heavy metals are actually necessary for fish (and humans) to survive. Zinc, for example, is a component of certain enzymes (eg carbonic anhydrase) that perform key biochemical functions, as Aquarian's fish nutrition experts well know. Zinc is therefore an important dietary component for fish, But at very high levels zinc and other heavy metals will be directly harmful to fish. Fish can deal with excess amounts of zinc in the water by actively excreting this metal from their tissues; but they can only cope with so much zinc, Hence, if the zinc concentration is very high then it will have toxic effects, causing gill damage and, in extreme cases, death. At sub-lethal levels zinc can inhibit the fish's growth and breeding potential. So, if your fish aren't getting 20 for size at the Show then perhaps there's too much zinc in the water.......it's possible.

Copper is another vital trace element necessary for life, but at high levels it too is potentially toxic to fish (incidentally, copper is an active ingredient of many traditional fish disease remedies and that's why it is important never to overdose with such remedies or to use them unnecessarily). The toxic action of copper on fish tissues is not fully understood but appears to be similar to that of zinc.
We know that copper is extremely toxic to certain aquatic invertebrates, affecting their respiratory systems. That's why copper based remedies must be used with extreme caution (ideally not at all) in mini-reef systems or tropical aquariums housing Freshwater Shrimps.

Even tiny amounts of heavy metals can harm fish. It has been suggested that fish should not be exposed 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).

Water hardness

This has a significant influence on the toxicity of certain heavy metals such as copper and zinc. Generally, metal toxicity falls with increasing water hardness (specifically, calcium content of the water). So if your fish are kept in soft water they will be more vulnerable to any heavy metals that may be present in their environment. Studies on Rainbow Trout illustrate the relationship between water hardness and heavy metal toxicity. In one experiment it was found that Trout held in water containing 10 mg/l calcium carbonate survived zinc levels up to about 0.5 mg/l, whereas those kept in much harder water, 300 mg/l calcium carbonate, survived up to about 2.5 mg/l of zinc -that's some five times higher.

Banishing chlorines and heavy metals

It is clear that 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 sub-lethal levels, these "contaminants" can affect our fish in various ways -causing respiratory difficulties, poor growth, poor breeding potential, and poor Show performance. The outward symptoms are not always glaringly obvious, or we may attribute poor performance to other causes:
"Mmm, I wonder why that brood of Tetras hasn't achieved good growth - maybe the infusoria culture wasn't right, or perhaps the water temperate was a bit low, or because there's an "R" in the month." Would our thoughts automatically turn to possible high zinc levels to explain poor growth performance in fish? - probably not.

Fortunately, we don't have to tolerate any of these tap water problems. Various reagents exist that deal swiftly with chlorine and chloramine disinfectants, and other chemicals can be used to neutralise any heavy metals that may be present. (Remember, heavy metals are simple elements so we cannot destroy them "matter cannot be created or destroyed" so the laws of physics tell us. But we can render them non-toxic to our fish).

The all-singing, all-dancing water conditioner

What is required is an "all-in-one" water conditioner that deals with chlorine-based disinfectants and heavy metals. Formulating the right blend of active reagents in such a product is the challenge. Well, Aquarian's new "Water Conditioner" achieves all these things, and more.

There are many water treatments already on the market but Aquarian's product has extra health benefits for our fish. "Water Conditioner" contains a plant extract that is known for its tissue healing properties.

The special extract helps in 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, hence is much more sensitive to cuts, infections and other traumas. Damaged skin - such as caused by fin-nipping, fighting, accidental abrasion on decor, net-damage and skin parasites (eg "Ich") is vulnerable to secondary bacterial or fungal infections. So the faster the fish's skin can heal, the better.

Following rigorous testing at the Waltham Aquacentre, the special "Water Conditioner" formulation has been given the "thumbs up". Hope you will try Aquarian's "Water Conditioner" at home and see the results for yourselves.

Healthy fishkeeping!

Need further information?

If you have any questions about any aspect of fish health and husbandry, then contact Peter at

The Aquarian Advisory Service,
PO Box 5059, Melton Mowbray, Leicestershire LE14 4ZN.

Or you can email him via the Ask for Answers service on the Aquarian website
or at aquaticsdoctor@aol.com

Last updated February 24, 2005