[redpaulhus]

There are lots of predatory fish that will do ok in that size tank.
However - do you mean predatory or do you mean piscivorous ?

Predators - fish that eat animals not plants - are easy in that size range - most dwarf cichlids, many gouramis or bettas, gobies or gudgeons, and killifish would fit the bill.

Piscivores - that's a bit tougher in that size range, in that most of the fish that would do well in such a small tank shouldn't be getting a diet of feeder fish (and most people who want piscivores seem to want to watch them kill feeders).