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Cloudy Water

Cloudy water in the aquarium can have several causes. The most likely ones are:

  • Debris from new substrate
  • Bacterial bloom
  • Chemical imbalance
  • Green water/floating algae

Debris from new substrate

This is the most likely reason for cloudy water in a tank which has only been set up for a day or two. Various gravel and sand substrates which are available for aquariums will tend to need some washing before they are used - the more they are prewashed, the less they will cloud the water (the gravel/sand can be washed by rinsing half a bucket full at a time, stirring it round and pouring away the water, repeat until the water becomes clear). If new substrate is the reason for the cloudy water, it should disappear in a day or two as the particles either settle or are filtered out.

Bacterial bloom

This appears as a greyish haze in the water. It is due to the explosive growth of bacteria, usually in a newly setup tank. It occurs because the bacterial population of the tank is not in balance with the level of waste nutrients. This will usually sort itself out as the tank matures - it should disappear more quickly if partial water changes (say 20%) are performed. Also, avoid overfeeding (very important in a new tank), to help reduce the waste levels. Remove any uneaten food, decaying plant matter, etc. Note that the bacterial bloom will not cause any harm to the fish.

Chemical imbalance

This may also cause a greyish-white haze in the water. The causes may be complex, but it is more likely to occur in harder water which contains more minerals. Using lots of chemical additives (other than a standard dechlorinator/conditioner) may also complicate matters. The use of an aquarium water conditioner may help by binding heavy metals, etc. Minimising nutrients in the water with regular water changes may also help. If you suspect that excessively hard and alkaline water may be the cause, investigate the pH and hardness of your tapwater and tank water.

A short term haziness could also be caused by bubbles of gas in the water. This is less likely to occur if the water has been left to stand and aerate naturally for a while, and warm up to room temperature.

Green water/floating algae

This problem will appear as a thick green cloud in the water, and can become so bad that it becomes impossible to see the fish - however, it will not do them any direct harm.
To help combat this problem:

  • Avoid direct sunlight falling on the tank.
  • Do not leave lighting on for more than 12 hours a day. Longer periods will tend to boost algae growth, rather than promote plant growth.
  • Minimise nutrient levels with frequent partial water changes.
Sometimes, a series of larger (about 30-40%) water changes every few days will cause this problem to disappear. Ultra-violet (UV) sterilisers as part of the filtration system will also destroy floating algae cells, as well as disease organisms. Another short term solution is to use a Diatom filter, which can remove very fine particles from the water and should cure the immediate problem. However, remember that the root cause (usually too much light/excess nutrients), needs to be tackled to prevent it returning.


Note that if you are seeing an excessive amount of general particles in the water, it may mean that the tank requires more efficient mechanical filtration to remove particles - overfeeding or a heavy fish load will make this more likely.

Aquarium stores often sell water clarifiers which help to clump particles together ('flocculents'), making them easier for the filter to remove. These may help to eliminate some of the above problems more quickly, but remember to address the possible cause, as well as the symptom.




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