How Does a Fish Swim?
A fish swims by curving its body from side to side, stretching or expanding their muscles on one side of their body, while relaxing the muscles on the other side. The swishing tail gives a powerful backward push on the water that moves the fish forward.
A fish hardly ever paddles itself along with its other fins. The fish uses its fins mainly to keep its balance and to steer itself up and down and side to side.
Fish use their back fin, called the caudal fin, to help push them through the water. The fish’s other fins help it steer. Fish have five types of fins. They are the pectoral, pelvic, dorsal, anal and caudal fins.
Most fish have air bladders, or swim bladders, inside their bodies, that provide buoyancy and make them float in the water. Some fish, such as sharks and rays, do not have air bladders. They must constantly be swimming in order to keep from sinking.