The ones who don’t teach themselves defensive tactics are food. The quick learners survive to make the next generation. There’s the quick, the clever, and the food. :eek:
Yay a fishthread! Must register and add my 2 cents:
The fish in Renee’s picture is a “Humphead wrasse” (Cheilinus undulatus). As the name says, it’s a member of the wrasse family, not a grouper (although it looks quite a bit like it).
The humphead grows to immense sizes (more than 230 cm (4,25 feet) and 190 kg (419 lb)) and is unfortunately quite endangered due to it’s slow growth and tasty flesh 
The groupers are all ferocious predators as someone guessed above.
You got most of the points in keeping big sharks in aquariums: Keep them well fed, keep injured and sick fish out of the aquarium, choose other species that can either keep out of the way or are too small to be of interest to the shark(s) in question. (Keeping in mind that the different kind of sharks are specialized in different prey species/types!).
Even keeping all of the above in mind, accidents do happen: a local aquarium here in Denmark once lost a valuable “Bonnethead” (small species of hammerhead shark) to one of the big “Sand tiger sharks” in the shark pool!
In schools.
Just for that I’d vote to have you sleep with the fishes, then I realized you do 
See, the problem with having Jets underwater for the Sharks to fight is that the ocean is three-dimensional. That means it’s hard to get the fish all on the same plane.
Not in Hollywood.
Fish! On a plane!