if personality of the fish is no big deal there are a bunch of obscure predators you could try, like crocodile fish (5 inches), african butterfly fish (4 inches), dwarf puffers (pea sized, roughly 1 per 5 gallons due to territory disputes), perhaps (not entirely sure but if it's a solo potentially plausible in a 30) a Leopard Ctenopoma etc...
Personally i'd get a 20 and go for a coldwater river tank if i could get the toys i`d require for them (hillstream loaches, white cloud mountain minnows, maybe a goldfish, ) but above are some more predatory options i could come up with