What makes the water in a fish tank turn cloudy?

What makes the water in a fish tank turn cloudy?

Breed Facts

Watching vibrant, colourful fish swimming around in the crystal-clear water of their tank can be enthralling and relaxing, and is part of the appeal of keeping pet fish. However, if the water in the tank looks cloudy or murky, this can not only spoil your view and make it harder to see the full vibrant detail of your fish and tank, but also indicates that there is a problem with the quality of the tank water-which can have an acute and negative impact on the health of your fish and any live plants you keep within it.

If you keep fish and want to make sure that your water stays clear and clean or are having problems with cloudy water and are not sure what is wrong or how to fix it, read on to learn more about what makes the water in a fish tank turn cloudy, and what to do about it.

Cloudy fish tanks

There are a huge range of different things that can make tank water cloudy, and sometimes the problem will resolve itself within a couple of days or even hours, whilst some issues can be more of a challenge to fix. The colour of the cloudiness can help you to determine what the problem is-white cloudy tanks are usually caused by different problems than green, for instance.

Also, before you begin investigating the water itself and trying to work out the problem, make sure first that it is actually the water than is murky or cloudy, and not just algae, dirt or scratches on the glass of the tank, while the water itself is clear!

Next, we will look at the most common causes of cloudy fish tank water, and how to fix them.

Poor filtration

If your fish tank tends to get cloudy within a couple of days of cleaning out the tank, the chances are that the tank’s filtration is not up to the task. All fish tanks need filtration-even goldfish tanks-and keeping your fish in a small, unfiltered bowl is both very bad for the fish, and likely to lead to cloudy water.

Make sure that your filter is sufficient to filter the volume of water in the tank, and also that it is all working properly and does not need the filter media replacing.

New filtration

Many types of aquarium filtration require “good” bacterial cultures that develop over time in the filter media to keep the water clean and remove contaminants and “bad” bacteria, and so if you have a brand-new filter or have recently replaced old filter media for new, this can lead to cloudy water while the new filter media becomes colonised with the necessary bacteria.

Rather than replacing old media for brand new, try to culture the new media with the old, to give it a head start and ensure it is capable of meeting the demands of your established tank.

Poor quality tap water

Water straight from the tap needs to be treated and conditioned with the appropriate supplements before using it in your tank, to neutralise chlorine and other chemicals that can affect the health of your fish. Even if you collect rainwater for your tank, the quality of it may be variable and it can easily get contaminated, so make sure that you are using the right water treatments in the right quantities before you add new water to the tank.

Algae blooms

Green, brown or otherwise murky water is almost always caused by algae blooms, which can happen if the tank is kept in direct sunlight, receives too much light, or otherwise provides the right environment for algae to grow out of control.

Keeping snails or fish that eat algae in your tank, regularly scraping the tank sides and ornaments to remove algae and potentially, using an algae treatment supplement can all help, as can of course ensuring that your tank is not getting too much light.

Water that is too old

If you don’t clean your fish tank out regularly and perform sufficient water changes, the water will soon look and smell bad, and affect the health of your fish. Make sure that you clean the tank and perform partial water changes regularly, and do not leave the tank until it is disgusting and then totally replace all of the water!

A new tank set-up that has not settled

New tanks need time to settle in before you start stocking them-a tank should be left at least a week or two before you add any plants, then longer again before you add your first fish, and new fish should be added gradually over time, not all at once!

If you set up a new tank and populate it too quickly, the filtration will not be able to keep up and there will not be enough cleaning bacteria established in the tank to maintain stable water quality, which can lead to a tank turning murky or cloudy overnight.


Too many fish in the tank will always compromise the water quality, as well as placing the filter under stress and causing your fish to be stressed out as well. The vast majority of domestic fish tanks and aquariums are actually overcrowded-fish need much more space than many people think, and so you may find that you have to get a second tank to ease the pressure on your first one if you have too many fish!


Overfeeding your fish is another common cause of cloudy water, as fish produce a lot of waste, and are also often permanently willing to eat however much is offered to them. Ensure that you are not overfeeding your fish, which can soon contribute to poor water quality and a cloudy tank.

Unrinsed gravel

Finally, one of the simplest explanations for cloudy water in a new tank is gravel that has not been rinsed properly before placing it into the tank! New gravel needs to be rinsed out very thoroughly and for quite a long time to remove dust and hard to see muck and debris on it, and if you are not vigilant about this, you may found that apparently clean-looking gravel turns the water cloudy as soon as you put it in the tank.



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