Green water is often referred to as "pea soup". Free floating single celled algae, grows at such a rapid rate that it turns the water green.
Water changes are not very helpful, as the algae spores can be found in almost every water source. The only other ingredients needed for a green water outbreak are high nutrient levels (nitrate) and intense lighting.
As with most nuisance algae, this single celled planctonic algae is capable of photosynthesis (light is used to create energy). During lighting periods oxygen is being produced during this process.
At night, the algae uses up oxygen from the tank, this can, if serious enough, suffocate the fish. Tanks with green water should therefore be very well oxygenated.
