After you set up a fish tank, you are sure to see algae growing in it. There is plenty of light, fertilizer in the form of fish droppings, and plants or glass for algae to cling to. You may see green strings of algae in the water or green spots growing on the glass. Your whole aquarium can turn green if you don't stop the algae. To prevent the problem, you can add algae-eating fish or microorganisms to the water. Covering the aquarium for a few days will also kill algae, which need photosynthesis to survive. Multi-celled algae grow into larger organisms through cell division. For one-celled algae, the purpose of cell division is