I'm guessing that you have arrived at this page because your tank water has suddenly turned cloudy. Worry not, cloudy water is fairly common in aquariums, both established and new. I will try and explain why this happens, what you can do and how you can avoid cloudy water in the future.
An aquarium containing living creatures is not just a fish tank full of water, it is a miniature environment just like any river or lake. When there is an unbalance in the system the first sign of trouble maybe noticeable by looking at your water. In a system that is working properly, your water should be nice and clear, if the water turns cloudy then this is a sign that all may not be working as it should be. We get quite a few people posting questions on our forum regarding cloudy water. In most cases the cloudy water has occurred in a newly setup fish tank. It takes approximately 6-8 weeks to cycle an aquarium and establish a bacteria colony in your filtration system. Until there is enough bacteria present in your biological filter, any excess ammonia that is not removed will encourage free swimming bacteria to start building up in the tank, this is what causes the cloudy water, we often refer to this situation as a "bacterial bloom". So if you are in the process of cycling your aquarium and your water turns a little cloudy, don't panic, keep testing your water for ammonia and nitrite, and keep up with the water changes. Eventually the cloudiness will disappear completely when your tank is fully cycled. If you are unfamiliar with cycling an aquarium them please read this article.
An excessive amount of algae can also make your aquarium look cloudy. It's normally green algae that occurs in aquariums that are exposed to too much light, whether that be direct sunlight, or aquarium lights that are left on for too long. High nitrate levels will encourage algae to grow so if you don't carry out enough water changes you may experience green cloudy water. If you are experiencing green cloudy water then carry out extra water changes, reduce feeding and cut down the amount aquarium lighting the aquarium receives during the day.
A very good example of when fish tanks often go cloudy is when people bring a goldfish home from the fair and put it in a bowl. This practice is hopefully quite rare now, but years ago, people didn't think that their goldfish needed any filtration. Of course all fish produce waste (ammonia) and without the means to remove this, the water will turn cloudy.
Your aquarium water may well look nice and clear when the lights are not switched on. However when you switch them on things look a little bit murky. It may not be a case that you have any problems such as a bacterial or algae bloom, it could be as simple as lots of bubbles in the water, especially if you've got a fairly strong filtration system.
Please be aware that the introduction of new substrate such as gravel or sand to an aquarium may well result in cloudy murky water. We always recommend cleaning sand or gravel before using it as substrate, however with all the will in the world there will always be a little bit of dirt or dust still contained within the substrate which is bound to cause a little bit of cloudiness in the water. This doesn't normally last for more than a day or two so let things settle if your water has turned cloudy after adding new substrate.
Below is a list of some basic mistakes that should be avoided if you don't want to wake up to a cloudy aquarium
One mistake many people make, normally newcomers to aquatics is to switch the filtration off at night, probably thinking that they are saving money. This is a major mistake that will completely wipe out your biological filter overnight. You will then have to cycle your aquarium over again. So never switch your filtration off for too long.
Another big no-no is washing biological media in the sink using tap water. I would say that 90% of newcomers have probably done this at one time or another. Water companies add chemicals to their water to make it safe for drinking. As soon as you pour it all over your biological media, you kill the bacteria straightaway. Again, you would have to cycle your tank over again.
Some medications contain chemicals that are very damaging to your biological filter. One of which is methylene blue which is often used to stop fungus growing on eggs. Always read instructions before adding medication to your aquarium.
Water conditioner should always be used when adding tapwater to your aquarium. Whereas small amounts won't pose a problem, a very large water change without using water conditioner could affect your biological filter, notwithstanding harming your fish.