there are many different kinds of fish. they live in fresh water in lakes and rivers, and in salt water in the oceans. some fish are less than one centimeter long. the largest fish is the whale shark, which can be almost 15 meters long and weigh 15 tons. almost all fish live in the water. a group of fish called the lungfish have developed lungs because they live in rivers and pools which dry up in certain parts of the year. they burrow into mud and aestivate until the water returns.