Algae are a large group of very primitive plants. Algae usually grow in the water, and the algae most people are familiar with is the green slime that grows on the inside of fish tanks. Algae can range in size from that familiar green slime right through to what is commonly called seaweed. Most algae grows in the water, but it can also be found on land too. Some algae grow with other organisms. Lichen is algae and fungi growing together. Some corals and sea sponges have algae growing inside them to provide them with oxygen and sugars.