What is the most dangerous aquarium fish?

I would think either the snakehead or the piranha though the piranha only attacks when there is commotion in the water.

The giant freshwater stingray kills more than 2000 people every year, and is definitely one of the most dangerous.

Bull sharks can live in fresh or salt water and have been known to attack people, although in some countries where there is plenty of the sharks natural diet available, attacks are rare.

There is a television series called 'River Monsters' that is fantastic if you are interested in the various (mostly freshwater) species that involve fatalaties.