Actually according to the Ontario Science Centre whether or not fish fart, is an ongoing debate. Some experts say that the digestive gasses of fish are consolidated with their food waste and expelled in gelatinous tubes which fish then eat. But, fish have been known to emit bubbles due to an excess intake of air. For instance the sand tiger shark purposefully gulps air into its stomach at the surface then farts it out at the other end so it sinks to its desired depth. http://www.ontariosciencecentre.ca/scizone/question/default.asp?teaserIdEN=380