My Fish Tank Water Has Turned Cloudy

Cloudy water, sometimes referred to as a bacteria bloom.  It has happened to a lot of us at some stage and is one of those questions that crops up quite often. I will try and explain this as easily as possible. There are several types of bacteria that are present in a fish tank. Some live in your filters and are attached to things like rocks and gravel, they need a surface to cling to. However there are some types of bacteria that can be free swimming and live in the water. These are called Heterotrophs bacteria and are the ones that we are concerned with regarding cloudy water . Heterotrophs are completely different to nitrifying bacteria that are found in you filtration system.  Contrary to popular belief, a cloudy tank does not always mean that your tank is going through a mini cycle.

So, first things first, test your water straight away for ammonia, nitrite, nitrate and pH  ( ammonia is more toxic with high-level pH) Then we maybe able to answer the question  "why has my water suddenly gone cloudy?" Firstly, establish whether or not you have turned you filters off for too long,  or if you have washed your filter media out in tap water.   Okay, developing cloudy water can happen in a well-established fish tank. Once again, cloudy water does not mean that you are going through a mini cycle. If you suddenly see that your water has gone cloudy, have a look at your feeding regime. How much food are you putting in a tank, there is a good chance that you are putting in too much food. The fish aren't eating all the food which means that the heterotrophs bacteria will increase accordingly and that is when you end up with a cloudy tank. It can also happen after a water change as well. Sometimes after vacuuming the gravel, nutrients are released and once again the bacteria multiplies to keep up with it. In certain cases, such as when the filtration system has been turned off, large amounts of ammonia will be produced in the tank water. When this happens, bacteria can adapt to actually consume the ammonia, this is when you may get a cloudy tank as well. If your water does suddenly turn cloudy, I would advise you to cease feeding straight away,  or if you are reluctant to do that, feed them very sparingly. Doing this would reduce the amount of ammonia the fish produces.  Your fish will be okay without food for a while.  You can then carry out regular water tests as well as regular water changes until things settle down again.

The overstocking of a fish tank often results in cloudy water. What you are doing is introducing more waste than the filtration can cope with.  Ammonia starts talking up in the tank water and this is when the water goes cloudy. In situations like this, your fish are at risk. You must carry out extra water changes and if necessary, use products like Prime that will convert the ammonia into a non-toxic form.

Let's briefly mention nitrifying bacteria. These are called  Autotrophic bacteria and live in your filtration system and are present on things like rocks and in the gravel.  Your filtration system is one of the most important parts of your tank set up and you cannot afford for it to go wrong. If it does go wrong, you are putting your fishes health at risk. Inside your filters you have what we call media. Now, this can consist of various substances such as sponge. When your fish tank has been running for a while, a bacteria colony will build up in the media. This colony is your best friend as it is what keeps your fish from dying. Your bacteria must be looked after. Autotrophic bacteria need oxygen. If you turn your filtration system for too long, they will be starved of oxygen and then die.   Most countries add chlorine and other horrible stuff to household water. If you wash your filter media out in water that has all these horrible substances in, your bacteria are going to be pretty upset. In fact, they are going to die. Once they die, you have got nothing to control the toxins that your fish produce. You will then have to cycle of your tank all over again. And if you have got a healthy population of fish, you could run into all sorts of problems.

So I will finish this chapter of by repeating, never wash media out in tap water and don't turn your filters off for too long. If you follow these two rules, you shouldn't encounter any problems


There is of course the chance that a bacteria bloom is not causing your cloudy water. Green algae can often make your aquarium water very cloudy, people often confuse this with filtration problems. However, this normally builds up quite slowly, so you will notice your water getting more and more cloudy as the days go by, rather than overnight. You may want to read up on what an algae bloom is on Wikipedia