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Aquariums & Ponds

Aquariums & Ponds

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.


Phosphates are a source of nutrients that contribute to extensive algal blooms within the aquarium, and can accumulate in the water at quite an alarming rate. Some regions (i.e. near farming communities) have naturally high levels of phosphate in their tap water, plus many dried fish-foods contain surprisingly high levels of phosphate, which soon build-up within the confines of an aquarium. It is not thought to be toxic to fish unless present in extremely high levels, but remember phosphates feed algae, and in turn, excessive amounts of algae can starve the aquarium of oxygen, which could spell disaster for your fish if not kept in check.


General hardness, or GH as it’s known, is the measure of (primarily) calcium and magnesium ions in the water, as well as those of carbonates and sulphates. These are often referred to as ‘dissolved salts’. You will find that GH is expressed as either parts per million (ppm) of calcium carbonate, or as degrees hardness (°dH).

As a general rule, water hardness follows these criteria:

0-4 °dH (0-70 ppm) = very soft

4-8 °dH (70-140 ppm) = soft

8-12 °dH (140-210 ppm) = medium

12-18 °dH (210-320 ppm) = hard

18-30 °dH (320-530 ppm) = very hard

So a low GH indicates softer water, and a higher GH, harder water.

Different species of fish prefer different GH levels and you should try to provide them with their requirements for best long-term health.


KH, or carbonate hardness, is sometimes referred to as buffering capacity.

This is in reference to the water’s ability to resist changes in pH. When the KH is low, it can cause problems to both fish and plants because the pH is liable to ‘swing’ i.e. change rapidly. If your KH is too low, the use of a buffering product will be beneficial, and if it is too high, consider using R.O. water for your water changes or add some aquarium peat into your filter.

Commonly Asked Questions

In aquariums and fish farms, you need to be able to differentiate between Carbonate Hardness (KH) and Total Hardness (GH).

Carbonate hardness (KH or CH) refers to the ability of the water to maintain a stable environment and subsequently, the health of fish. The carbonate hardness is the 'buffering' ability of the water. To determine this on a photometer you need Palintest Alkalinity tablets, known as Alkaphot (part code AP188).

General hardness (GH) refers to the 'hardness' of the water caused by calcium and magnesium ions. Different aquatic species prefer different environments, some prefer 'hard' water and some prefer 'soft' water. The hardness of your water is often dictated by the source of the water. If it's from groundwater it tends to be harder, if it's from surface water it tends to be softer. To determine this on a photometer you'll need our Hardicol reagent (part code AP254).

Sometimes the units dH are used. This means "degrees hardness''. Most test kits define results in terms of ppm CaCO3 means 'parts per million equivalent to that of calcium carbonate' - it doesn't mean that the hardness is in this form.

1 unit dH equals 17.8 ppm as CaCO3.

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