Why do goldfish stay still in the bottom of the tank?

Goldfish have several different fins that assist them in moving from place to place. Starting from the back, the caudal fin moves in a left to right motion to propel the fish through the water. The anal and ventral fins are for steering. A fish's pectoral fins are also used for steering, but in some fancy breeds these fins are also used for propelling the fish, such as with the Ryukin. Last but not least, the dorsal fin on the back of the fish is used for stopping. The fin is fully erect when stopped, and lower on the body when in motion. Fun fact, a goldfish cannot swim backwards.
Goldfish use their tail and fins to swim through water. To learn more, see the related link.