Staying Afloat

The group of fish which evolved with bony skeletons also evolved a way of staying afloat without the constant need to keep swimming.

They developed a swim bladder which is a bag of air that can be filled or emptied by diffusion of gases or by expelling the gases direct 

This way a fish can control its buoyancy and level in the water.

Compare a picture of a shark and a fish.

What other differences might help to make the fish such a manoeuvrable swimmer?