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Water Quality

Along with stress, which goes hand-in-hand with bad water quality, excessive pollutants in the water are one of the most common causes of fish losses and general bad health in the aquarium. Put in crude and simple terms, fish are basically large water-filled membranes living in a water-filled environment. This means that substances in the water can easily pass through the fish's body (membrane) and affect its internal organs. A large amount of a fish's energy is concentrated on keeping toxins out of the body and an excess of toxins can cause serious health problems either by direct effect on the fish's body, by the fish's immune responses, or simply by physiological stress placed on the fish.

Ammonia & Nitrite

The direct effects of ammonia and nitrite are described in the disease index but both are very harmful to fish and result in ill health, stress, and death in some cases. The stress caused by high levels of these toxins will weaken fish allowing disease pathogens to rapidly take hold. Excess ammonia and nitrites can occur in new tanks which have not been matured, through overfeeding, overstocking, unnoticed death and decomposition, or when the activity of filter bacteria is disrupted (most commonly through extended power-cuts, filter stoppage, replacement of media, or cleaning of media in tap-water)


Nitrates are the end result of the filtration process and are usually removed through water changes or the use of 'chemical media' such as activated carbon in the filter. For many fish, gradual increases in nitrates are tolerated up to very high levels without any noticeable effects in health or behaviour. This is a false impression however and high nitrates do have a serious effect on health. Under high nitrate conditions a fish's immune system is weakened and if diseases occur, they will spread rapidly and will be very difficult to treat. A common problem encountered with high nitrates is when new fish are introduced to the aquarium; whilst the existing fish may have gradually adapted to increases in nitrate levels, the new fish will be suddenly transferred from low-nitrate water to high-nitrate water. This transfer is a huge shock to the fish's body and usually results in disease and/or death, if disease occurs the existing fish (which otherwise may have seemed perfectly healthy) are likely to also become diseased very quickly. This situation is often blamed on the new fish, as the aquarium and its fish may have appeared perfectly healthy for months and only when new fish are introduced does disease suddenly affect the whole aquarium.

Fluctuating conditions

In the wild, although water conditions do fluctuate daily and with the seasons, the fluctuations are gradual and predictable, allowing fish to steadily adapt. In the aquarium, fluctuations of water parameters in a short period of time cause severe stress and weaken the fish's immunity to disease. Common fluctuations that can occur include the following:

Temperature - Caused by incorrect heater sizes, broken heaters, lack of flow around heaters, spherical (bowl-shaped) aquariums, positioning near heat sources, extremes of outside temperatures.

pH - Caused by a lack of water hardness (buffering), pH can fluctuate between day and night in water with little buffering due to differences in oxygen and carbon dioxide production between night and day. Buffering naturally diminishes over time in the aquarium if sufficient water changes are not carried out.

Lighting - Although not part of water quality, sudden changes in lighting such as those caused by aquarium lighting being switched off when there is no other light source (i.e. from bright light to pitch black) can be a contributing stress factor.


Not all pollutants can be easily tested for and there are many chemicals and substances that are poisonous to fish at certain levels. Toxins can be introduced to the aquarium through the use of chemicals such as cleaning agents, or airborne from cigarette smoke, air fresheners, scented candles, deodorants, and other such sources. As well as avoiding the direct use of any chemical not designed for aquarium use when handling aquarium equipment, it is also important to avoid the use of containers which have previously held chemicals when cleaning equipment or carrying out water changes.

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