Water Quality & Testing

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Water quality and water testing terms such as ppm, ammonia, nitrites, nitrates, hardness, pH, GH, KH, dGH, and the fact that all of these are interconnected seems like a whole laboratory and a degree in chemistry may be required.

Fish tank water being tested

The subject of water quality and testing can be as simple or as complex as desired. For most aquariums, water quality and water testing is far simpler to understand and carry out than it first seems.

Water quality factors affect the health and vitality of your fish, and all are also invisible to the eye in the aquarium. For this reason, crystal clear water should never be taken as proof of a healthy aquarium.

Incorrect water quality is the prime suspect in virtually all algae and fish health related problems and regular water testing is the easiest way to prevent any disheartening and potentially expensive downfalls. There are a number of methods of water testing but first it is vital to understand what you are testing and why.

Ammonia (NH3)

The food that enters your aquarium is the source of virtually all waste in the aquarium. As the fish digest food, they inevitably produce waste matter, a large proportion of which is ammonia. Ammonia, even at low levels is toxic to fishes and other forms of aquatic (and terrestrial) life. The primary function of the filter in your aquarium is to house bacteria, which convert ammonia into nitrites.

A suitable established filter should continually convert ammonia so that the quantity left in the water is well into safe levels. It is important to test regularly for ammonia in both new and established aquaria.

In new aquariums, the filter will not be fully established and if the amount of waste entering the aquarium exceeds the capability of the filter, ammonia levels will rise. In an established aquarium, an un-noticed death or damage to the filter by power cuts or incorrect cleaning will also cause rises in ammonia. A sudden, un-noticed rise of ammonia can cause a significant number of deaths, occasionally wiping out an entire aquarium.

Ideally, ammonia levels should be below 0.1ppm (ppm = parts per million) at all times. It is likely that slight fluctuations will occur in new aquariums, as long as the level is not above 1ppm for more than a few days, the first hardy fish in the aquarium should be able to cope. If ammonia levels do rise, an immediate 10-20% water change should be carried out, ideally at the same time as a gravel clean, and feeding should be stopped for a few days. This will dilute the ammonia content and reduce any near-future increases. Some chemical additives or specialised filter media are designed to remove ammonia quickly and are well worth using to quickly reduce ammonia related problems.

Nitrite (NO2)

Once the bacteria in the filter has converted ammonia, it produces nitrites, which are still highly toxic to fishes but less toxic than ammonia. Because nitrites come from the same source as ammonia, the same causes can be identified for increases of nitrite. A new aquarium with an un-matured filter, too much feeding, lack of gravel cleaning or a breakdown of the filter by power cuts or incorrect maintenance may all cause increases of nitrites.

Nitrite levels should also be at around zero but some hardy fish will cope with extended periods of up to a week or more with nitrite levels approaching 4ppm. The sensitivity of fishes to nitrites depends largely on the species of fish but in all cases, exposure to unsuitable nitrites will cause damage.

The remedy for high nitrites is also the same as for ammonia, an immediate water change and gravel clean along with a reduction in the feeding should quickly reduce nitrites. Again, additives are also available to quickly absorb and remove nitrites.

Nitrates (NO3)

The final process of the bacteria in the filter is to convert nitrites into nitrates. Nitrates are far less damaging than either ammonia or nitrite and only become toxic at high levels. Rising nitrates however, can reduce a fish's immunity to disease over long periods of time. If the aquariums nitrate levels do become high, the existing fish may seem healthy but any new fish may succumb to disease quickly, and also pass diseases to the existing fish.

Most hardy tropical freshwater fish will live healthy at nitrate levels as high as 50-100ppm, In some cases, hardy fish can survive for years at levels above 250ppm, although they will never be at full health and vitality.

To keep a healthy aquarium, nitrates should be kept below 50ppm. This can sometimes be tricky as some tap water sources may contain nitrate levels this high. In cases where tap water levels are high, the addition of live plants and / or nitrate absorbing filter media to the aquarium should effectively keep nitrates low.

In cases of high nitrates, a series of small water changes should quickly reduce levels. Regular water changes will continually keep levels low. If they do not, it is likely that either your tap water contains high nitrate levels or the fish are being continually fed too much food.


For many new fish keepers, pH is a subject that can be avoided without too many problems as long as they trust the advice of a good retailer without question. pH however, has a large effect on the health of fishes. This is because different fish prefer different pH levels, and also that pH levels vary in tap water depending on which area of the country you live in.

As a very basic description, pH is a measure of how acidic or alkaline the water is and depends on the ratio of hydrogen ions in the water. It is not too important to understand exactly how pH works, but suffice to say that water with a pH below 7 is acidic and water with a pH above 7 is alkaline, pH 7 is considered as neutral.

The ideal pH level for most tropical fish normally lies somewhere between pH 6 and 8 and depends on where they originate in nature. Fishes from alkaline waters, such as the rift lake cichlids of Africa, prefer water with a high (alkaline) pH of around 8-8.5. In contrast, many tetras which come from acidic waters such as those of heavily vegetated streams and rivers in the Amazon basin, prefer a low (acidic) pH of around 6-6.5. Apart from issues of compatibility, these two groups of fish would be difficult to provide ideal conditions for in the same aquarium.

The majority of aquarium fish will live happily in water with a pH between 6.5-7.5. It is only when the water in your aquarium falls outside of this range that you may be forced to either alter the water or stock only the fish which suit your pH level.

pH changes

Whatever the pH level of your aquarium, it is vital that it remains stable and never changes drastically over a short period of time. Sudden changes can occur in aquariums with little water hardness (see water hardness below) or when an incorrect source of water is used, such as pure rainwater.

Over time, pH will naturally drop in an aquarium because the wastes produced by fishes and wood are acidic. The effect of long term drops and the risk of sudden changes can be prevented simply by carrying out regular water changes. Water changes will continually replace the minerals that 'buffer' the pH level and prevent changes.

Altering pH levels

It may be required to alter the pH of your aquarium water or water source to accommodate the needs of the fish you wish to keep. There are a number of ways to do this and seeking advice from a good retailer is well recommended. Changing acidic (low pH) water into alkaline (high pH) water is relatively easy as it involves the addition of mineral substances. Calcareous (calcium containing) rocks and substrates can be used for this purpose, as can specially made chemical additives.

Changing alkaline water into acidic water is a little trickier as it involves the removal of substances. 'Natural' methods include the use of bog-wood or peat based filter media which both release acidic compounds causing the removal of alkaline substances. Chemical treatments and additives can also be used to lower pH but these should be used with great care and good advice, incorrect use can result in unwanted and difficult side effects.


Water can be termed as hard or soft depending on the amount of minerals and salts it contains. Water with few minerals is considered soft, whilst water with a high quantity is considered hard.

Water hardness is closely linked to pH. Hard water with many minerals has the ability to 'absorb' acidic substances and so prevents drops in pH. This is why hard water often has a high (alkaline) pH whilst soft water often has a low (acidic) pH. As with pH levels, different fish prefer different hardness levels.

In most cases, for new fish keepers, hardness is relatively unimportant and as long as regular water changes are carried out, the aquarium should maintain a suitable hardness level. It is worth noting though, that water with very little hardness may be prone to pH fluctuations, which can be damaging to fish.

Water with very high hardness prevents pH fluctuations but also makes conditions difficult for aquarium plants, which may not be able to absorb nutrients easily in hard water. It is worth testing for hardness simply to monitor gradual rises or falls, in case of potential problems caused by high or low extremes.

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