Gone Fishin’!

I’ve read up that a few fish (including the Atlantic tarpon) can supplement their oxygen levels by taking in air from the surface of the water if oxygen levels are too low. Is this unique in their anatomy? And what is stopping other fish from doing the same?

Atlantic tarpons are facultative air-breathers, in well-oxygenated waters they are able to meet their oxygen requirements without breathing air.

When swimming in oxygen-poor water, they can breathe air from the surface using their swim bladder as a primitive lung. This adaptation allows them to survive in water with low dissolved oxygen concentrations such as commonly encountered by juveniles in hot, stagnant mangrove marshes.

However, after several unsuccessful attempts to reach the surface they have emptied their swimbladders and become negatively buoyant until allowed access to the surface again.

Many fish can breathe air via a variety of mechanisms and it is primarily of use to fish that inhabit shallow, seasonally variable waters where the water’s oxygen concentration may seasonally decline.
Chapter Respiration reports more details:
http://en.wikipedia.org/wiki/Fish