It is the free floating planktonic single cell algae growing at a phenomenal rate that turns the water green, often referred to as "green water". The cause is always the same, excess light and nutrients (nitrates), often resulting from overfeeding and uneaten food. Obtain a nitrate reading to confirm this.

Whenever a new aquarium is set up a number of cycles occur, some you probably know of, such as the nitrogen cycle. However, a phosphorous cycle also occurs, which may be stimulated if phosphate buffers are overused when excess nitrates are present, so by adding a phosphate remover to the aquarium filter the situation is compounded.

Performing water changes can alleviate the situation to some degree, however, if steps aren't taken to remove the nutrients and /or the excess light that are causing problem, the water changes are providing little help in the long term. Algae spores will thrive in nutrient rich water, an algae bloom can become so severe that the content of an aquarium can vanish in the green water.

Algae are a form of plant life and will consume oxygen at night. If algae bloom is severe, adequate oxygenation should be provided during the lights off period at night-time, it is possible for a severe algae bloom to deplete the tank of oxygen.

So if algae are a form of plant life you will be right to think that it requires exactly the same conditions as plants to survive, i.e. light and nutrients. Therefore to rid yourself of the algae you need to remove the things that it needs to survive.

Dying organic matter will create just the right nutrients that algae need for survival, so the filter should be rinsed more frequently during an algae bloom, eliminating some of the decaying matter. Installing a larger internal filter in the aquarium will go one step further to fixing the problem.

Other possibilities that can help to rid the problem include vacuuming the gravel of all debris, reducing the light in the aquarium and reducing the length of time it is on. Be sure not to overfeed the aquarium, the fishes will only eat the amount of food that fills them.

Live Daphnia are minute crustaceans that are actually a favourite food of fishes. If you can find a supplier of live daphnia get some and introduce them into the aquarium, they will eat off your green water in no time that is if the fishes don't eat the daphnia first. Live plants are always a good option because they absorb the nutrients that the green algae will otherwise thrive on.

In summary, the only way to prevent or solve an algae bloom is by controlling excess nutrients.