From this story on Wired:
Professional diver Scott Gardner has captured what are believed to be the first images of a wild fish using a tool. The picture above, captured in Australia's Great Barrier Reef, shows a foot-long blackspot tuskfish smashing a clam on a rock until it cracks open, so the fish can gobble up the bivalve inside. Tool use was once thought to be exclusive to humans, and was considered a mark of our superior intelligent and bulging brains. In recent decades, though, more and more animals have shown an ability to work with tools and objects.
Elephants pick up branches with their trunk to swat flies and scratch themselves, a laboratory crow improvised a hooked tool from a wire to extract an insect and primates use sharpened sticks as spears, rocks to smash nuts and sticks to poke into ant nests. Tool use in fish, however, is much more rare, and there's never been any photo or video evidence to prove it -- until now.