You guys think too small. I'd take the entire ocean and stick it in my fish tank if I could Or at least the entire Great Barrier Reef...
But really, a school of Mantas, a pod of dolphins, lots of baitfish, ocean sunfish, and (in a different tank) some gargantuan squid (bigger then giant squid, they live in the antarctic ocean under the ice, and need to supercold water due to it's high oxygen content).
I'd go with a self maintaining evosystem. With bait fish and prey.
A big school of anchovies.
Then a smaller school of yellowfin tuna, wahoo, salmon.
Oh and of course Jessica Alba. She stared in Good Luck Chuck as the penguin sitter.
Squid
Dolphins
A pretty great white shark
Manta rays
A pretty great white shark
Shrimps (guess they would be very small if shrunken haha)
Does the great barrier reef count? If so yes.
Turtles
More turtles
And a pretty great white shark like Bruce. -fish are friends. Not food-