Fish accomplish primary movement by relaxing and contracting bands of muscles around the centers of their bodies on opposing sides. This causes their tails to move back and forth.
Fish have five different kinds of fins, including caudal or tail fins and pectoral, dorsal, anal, and pelvic fins. Fish primarily use all except caudal fins for stabilization and steering in the water; however, other fin types can help with propulsion when the fish is fleeing danger.
Some species such as sharks and tuna can swim up to 50 miles per hour in short bursts. These great swimmers have cruising speeds of around 5 to 10 miles per hour on average. A number of different species have the ability to swim backward, although fish of this type are rare. Fish in the eel family are most likely to engage in this practice.
There are a few fish that swim vertically instead of horizontally in an upright position. A seahorse is one example of such a fish. The shrimp fish found only in the Indian Ocean also swims in schools vertically. In addition to these, there are catfish native to Africa and a variety of deep-sea fish that also swim or rest in a vertical fashion.