If you have a new aquarium set up or have recently added something to the tank resulting in cloudiness of the water, don't panic, it is merely an indication of dirt in the gravel and you should either wait for it to settle or get absorbed in the external filtration. A yellow or brownish tint means that you have an overabundance of organic matter in the system and a carbon additive will sort this out.
The actual term cloudy water' refers to two different types:
Green water. This is the stuff that makes your aquarium look really murky. You can barely see the fish as they swim about in a dingy gloom. The cause is an algae that grows at an alarming rate from the result of overfeeding your fish and an excessive use of lighting. Changing the water doesn't really help as the spores are mostly present even in the new change of water. Adequate oxygen must be provided for your tank while you are going through the process of eliminating the algae. To control its growth one way is to vacuum the gravel as a lot of rotting matter will be hidden to the naked eye and rinsing the filter a little more often can help. Cut down to the barest minimum the amount that you feed your fish and keep the tank out direct sunlight in fact keep light to a bare minimum.
White cloudy water: This can be caused by a number of factors. It can be triggered by replacing the tank with new gravel or cleaning all the filters at once. One thing to remember with an aquarium whether it be changing a process or introducing more fish or to rectifying a problem... the best way to do it, is gradually. Any sudden change in your tank's self contained environment can cause a mini eco disaster. Cloudy water can appear weeks or months after a new aquarium has been set up , the cause is usually due to bacterial bloom. As the new aquarium goes through the initial break in cycle, it is not unusual for the water to become hazy. It will take a little time to establish bacterial colonies that are able to clear wastes from the water. With patience the cloudiness will eventually abate.
Green water and white bloom is a worrying and time consuming problem that can be easily avoided by cutting down the amount of nutrients in your tank water. Situating your aquarium out of direct sunlight when setting up is good foresight and a sensible use of lighting is recommended. When feeding your fish watch out for overfeeding and wastage. One tip is to always feed your fish at the same time each day so that they feed immediately and you can see when they have been adequately fed. Turn all lighting off at night as your fish will need a natural day and night routine anyway. A period of 12 14 hours a day is sufficient.
If all else fails and you need a little help you can buy additives for your tank water that can help speed the process up. These products can be extremely helpful in helping to condition your water in a much shorter time, however it is important that you still follow the instructions to decrease the nutrients in the water.
Sera Aquarium Handbook 2