Room 2, of Woodlands School, asks :-

Why does the water in our aquarium eventually turn green?

Paul Broady, an aquatic botanist at the University of Canterbury , responded.

There is a group of simple plant-like organisms, members of which are found practically everywhere there is light, at least a little water and some fertiliser. These organisms are called 'algae' and there are tens of thousands of different species living in the oceans, in lakes, ponds and rivers and in soils.

An aquarium, whether it is filled with freshwater or seawater, is a perfect place for algae to grow. The conditions are generally warm, light is provided by electric lamps, there is lots of water and fertiliser is constantly being put in the water. This fertiliser is the fish food and the waste products produced by the fish after digesting the food.

Microscopic algae measuring between one thousandth and one tenth of a millimetre in diameter can grow rapidly under these conditions. Many of these are green in colour, although if you look carefully you will see brown, blue-green, orange and even blackish crusts and slimes on rocks, leaves of large aquatic plants and the glass walls of the aquarium.

The best way to prevent the growths is to constantly remove the wastes produced by the fish. You can also try by replacing the old water with clean water quite frequently. The glass can be scraped clean easily too. However, it will be a constant battle as there will always be small numbers of algae even in clean water and also their spores will drop out of the air into the water. These cells act as a source of the dense green growths which often follow.

On a more serious note, the problem caused in small aquaria is similar to that which can be produced by fishfarms in our coastal waters. The rich nutrients in those waters can stimulate the growth of toxic algae which can kill the fish. We will have to avoid this problem by careful management as fish-farming becomes more widespread in New Zealand waters.