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 sounds 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.

I think it's probably worth mentioning that a bacterial bloom can occur in not just new tanks, but established aquariums as well.  People sometimes experience cloudy water after they have cleaned their substrate.  Gravel especially can harbour lots of nutrients that when released into the water after a gravel clean can sometimes encourage free swimming bacteria which in turn will cause cloudy water.  However, you mustn't worry if this happens as it will probably disappear as quickly as it came.  Another cause of a bacterial blooms is if you are a little bit overzealous when cleaning your filtration system.  You must take great care when washing your media, if you remove too much bacteria then you may experience cloudy water as it will take a few days for the bacteria to catch up with the bio load.  One very important thing to remember when either stocking your aquarium for the first time, or introducing new fish is will your filtration system be able to cope with the extra bio load that those fish will create?  If you have cycled your aquarium using a few small fish then suddenly introducing a large Oscar maybe a little bit too much for the filtration to cope with, again this is when you may notice your water clouding up.  So be careful when setting up your aquarium for the first time, don't add too many fish to begin with.  The same goes with overstocking an aquarium, if there are too many fish for your filtration system then you will probably have permanent cloudy water.

Green algae can also be the cause of cloudy aquarium water. Green algae is not directly harmful to your fish, it just looks unsightly and may mean you can't see your fish properly. Preventing algae from growing in your tank is not difficult. Just follow a few simple rules. Don't leave your lights on for too long, eight hours a day is probably the recommended period for lighting your aquarium. Don't put your tank in direct sunlight, this is a sure way of encouraging algae growth. Carry out regular water changes as high nitrate levels will encourage algae to grow.