If you really want a shark you could get a silvertip shark. It's a brackish fish that can live in a marine environment if acclimated slowly. They can be put in a 75 gallon, but that would be it as they do get about a foot long.
You obviously wouldn't want to buy the fish, and put them in your tank right away; as that would surely kill the fish. I would do some research on this as it can be very difficult, and time consuming.
- 2 Percula Clowns
2 Emerald Mithrax Crab, 5 Nassarius Snails, 3 Astrea Snails, 1 mexican turbo snails, 1 Chocolate Chip Star, 1 Peppermint Shrimp, and 27lbs of LR
2lbs tonga branch and 25lbs fiji rock
Maiden's Hair, Calerpa, and Red Kelp macro algae.