The back-and-forth movement of the tail fin provides the goldfish with forward motion. The fish literally pulls the tail from one side of its body to the other. By going back and forth, the tail pushes the water behind it, thus pushing the fish forward. By bending the tail appropriately, the fish also steers itself.
The dorsal fin keeps the fish right-side up, and the pectoral, ventral and anal fins help to thrust and steer, as well as stop. The pectoral fins are used for tight turning and hovering.
The fish stops by reversing the tail motion, quickly. All other fins immediately become rigid. Sometimes the pectoral fins are instrumental in backing up and are at times used when a quick or sudden stop is needed, much like a thruster rocket on a spaceship.
How Do Goldfish Breathe?
Goldfish, like all fish, require oxygen to live. Since they live in the water they do not breathe as we do. Instead of lungs, they have gills. These organs extract oxygen from the water and expel carbon dioxide from their own system, much like other vertebrates. As the fish swims, it opens its mouth, taking in vast amounts of water. An oral membrane automatically closes off the water from going down the proverbial wrong pipe.
The water passes through the gills, which in the goldfish are located on either side of the head. The gills are filled with tiny membranes.
As the water passes over these membranes and filaments, oxygen is exchanged for carbon dioxide. As the blood passes through the filaments, they oxygenate the blood while releasing all the other unwanted gases. This is called osmosis. A minute amount of breathing takes place through the skin.
When there is insufficient oxygen in the water, fish rise to just below the surface, their mouths actually protruding from the water, gulping for air. These fish are actually trying to avoid suffocation. This circumstance most often occurs in the well-known "goldfish bowl."
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