How to Maintain Aquarium Water Quality

Water quality is affected by two main factors: firstly, contaminants (organic and inorganic) which it contains when drawn from the tap, and secondly those which enter it while it is in the aquarium. Both inorganic and organic contaminants are involved.

Inorganic contaminants

Tap water may contain small quantities of various metallic salts, either naturally or as the result of environmental pollution. It may also become contaminated with lead or copper from domestic pipework, and for this reason water from the hot water cylinder, or water that has stood in the pipes for some time, should not be used.

Chemicals used by water companies to kill bacteria (chlorine and chloramine) and crustaceans are also toxic. Water companies are required to provide water with safe levels of contaminants, but unfortunately what is ‘safe’ for humans may be lethal to delicate fish.

aquarium water quality How to Maintain Aquarium Water Quality

Purifying tap water

The simplest way of making tap water safe for hardy tropical fish is to add a conditioner to remove, or nullify, the effects of chlorine and chloramines; if only chlorine is present (ask the water company) then it will dissipate if the water is allowed to stand overnight, preferably with aeration, or can often be driven off by running the tap hard into a bucket. Some conditioners also nullify other toxins, e.g. metallic salts; not all conditioners remove all toxins, so always buy one appropriate to your needs.

To improve tap water still further, we can use special resins (available from aquatic dealers), usually in special tap water treatment filters, to remove nitrates, phosphates and sulphates – all of which are less than conducive to fish health and can encourage unsightly algae in both freshwater and marine aquaria.

In-tank pollution

In-tank inorganic contamination may derive from unsuitable decor or equipment, or the accidental influx of chemicals (such as cigarette smoke, insecticide sprays, aerosol furniture polish, paint fumes etc.). While to our knowledge no smoker has ever been implicated in the demise of any fish (at least through smoking!), most other extraneous toxins are (often quickly) lethal, and prevention is the only remedy.

Organic contamination

Organic contaminants may be present in tap water, usually derived from agricultural fertilizers or resulting from the impossibility (tit least affordably) of totally purifying recycled water. Aquarists whose water comes from rivers, or river-fed reservoirs, should be particularly vigilant. Artesian supplies and rain (or spring) water reservoirs are generally more pure.

Organic pollution within the aquarium comes mainly from the metabolic processes of its occupants (plants and animals). This pollution is controlled by the nitrogen cycle, and an understanding of the latter is essential to the aquarist.

The nitrogen cycle

The nitrogen cycle is the means by which waste products are dealt with biologically by Nature, both in the wild and in our aquaria.

The first part of the cycle deals with the decomposition of the remains of plants and animals, and the excreta of the latter. These are initially broken down into highly toxic ammonia (also excreted by fish) and associated compounds, which form the diet of a genus of bacteria called Nitrosomonus. These bacteria convert the highly toxic ammonia to slightly less toxic nitrite in the first stage of the cycle.

Next, another genus of bacteria, called Nitrobacter, convert nitrite into nitrate. Nitrate is not particularly harmful to most fish in reasonable quantities, though tolerance levels vary from species to species. It may be used in part by plants as natural fertilizer.

Both Nitrosomonus and Nitrobacter bacteria are aerobic, i.e. they need oxygen to survive, breed and perform their tasks. In Nature – and in the aquarium – they inhabit any suitable surface such as rocks, wood, sand, gravel and plant leaves.

Ammonia, nitrite and nitrate cannot be seen in the aquarium water, but test kits are available to test for the presence of all three.


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