There are two primary reasons that an aquarium will have cloudy green water. However, please note that it is nearly impossible to diagnose the exact cause of your predicament without further information about your fish tank (water parameters, feeding schedule, etc…). Here is some advice to get you started in the correct direction.
New Tank Syndrome
When you first set up an aquarium it has to go through a process most commonly known as “cycling” to fish hobbyists. During this period your aquarium is establishing a strong colony of bacteria that will in turn prevent the ammonia, nitrite and nitrate levels from rising to dangerous levels. Within the first couple weeks you should experience some very cloudy water that may or may not be tinted green and have an odor.
What You Should Do
If you are experiencing cloudy green water because your tank is less than two months old you should not have to take any action because this is an entirely normal process. However, you should limit the amount of new fish you are introducing during this period so that you don’t strain your budding bacteria colony. Just try to keep in mind that the cloudiness is a necessary step- Do not use a water clarifier in attempts to rectify this phenomenon.
An Excess of Nutrients
The other common reason why an aquarium will have cloudy green water is because there is an excess of nutrients. This can be caused by a number of things, however over feeding and over stocking are generally the culprits. Over feeding can explain the green color of the water, which is actually a form of free-floating algae- It feeds on the excess nutrients which is why it is thriving. Cloudy water after the tank has already been established is often the result of a common stocking error. When too many fish are added at one time there is an increased stress on the bio-load, thus resulting in cloudy water as the bacteria colony tries to play catch up with the higher amounts of ammonia from the fish waste.
What You Should Do
The first step you should take when you suspect that your aquarium has cloudy green water because of excessive nutrients is to examine your feeding schedule. The majority of fish do not have to eat every single day, multiple times a day- Fry are an exception to this rule. For example, many species of cichlids do best eating every other day or so, as do bettas who are being kept in smaller aquarium. At the very least limit your feedings to once a day. You can also prevent cloudy green water from occurring in an established aquarium by introducing new fish in small numbers. Ideally, you only want to add two or three fish at a time and then wait a week or so before adding any more. This allows your bacteria colony to slowly adjust to the increased amount of fish waste which should eliminate the formation of cloudy water.