Cloudy Fish Tank – What Can You Do?

If your fish tank has gone cloudy, don’t despair, it’s quite common and is unlikely to be anything that is harming the fish. Fish don’t seem to mind; in fact it’s the tank owner who gets more upset simply because all their efforts are disappearing into a white fog.

I’m going to assume that your tank has been set up for a while and rule out the cloudy fish tank water that can happen if you don’t rinse the substrate thoroughly before setting up.

That really leaves a couple of options depending on the color of your cloud. If it’s white it is most likely caused by a bacterial bloom.   This is a sudden increase in bacteria suspended in the water which can give the cloudy fish tank effect. This often happens days, weeks or even months after the tank is first set up.

When the nitrogen cycle kicks in you start to form two types of bacteria that deal with the waste from the fish. Depending on their numbers they can cause a cloudy fish tank. As the bacteria run out of food their numbers will decrease and the cloudiness will go but this may take several weeks.

You can short cut the process by cleaning up the tank, removing all decaying plant matter and vacuuming the gravel to remove uneaten food. Cut down on the feeding to perhaps every other day for a while. Do a head count and make sure that your tank isn’t over stocked. Remember fish grow and you won’t be the first aquarist to get caught out like this.

Bacteria grow on decaying plants, left over food and fish waste. Reduce these and you will reduce the bacteria.  If the cloud is green, congratulations you have green water – caused by an algae bloom.  The causes are similar to what causes a bacterial bloom. If there is too much waste in the tank, your bacteria will convert it into nitrates. If you let the nitrates build up you can end up with an algae bloom. Same rules apply, clean the tank, do a series of partial water changes to reduce nitrate levels and you could also consider planting some fast growing plants to compete for the nitrates.

It is also worth checking phosphate levels. If they are too high this will favour algae and encourage them. So will too much light. It is often tempting to leave the lights on longer so that you can benefit from your fish.

Twelve hours is the recommended time and if you exceed this you will be encouraging algae and end up with a cloudy fish tank.  Most of this comes down to tank maintenance. If you have regular tank cleaning routine then most of the time your tank will be fine. Occasionally nature has a way of showing us who is really in control.

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Nick North is an aquarium enthusiast with nearly 20 years experience of keeping freshwater tropical fish. He has set up a website to help anyone who is new to the freshwater aquarium hobby. Check it out at