from my experience, the puffers i bought nibbed my other fish's tails and fins. these nasty fella would slowly creep behind its victims and snatch a bite out of the tail and dorsal fin. most of my not-so-fast-moving fishes tails and fins became 'kontot'!! damn nasty. i throw away all of them.
they are good if you want to get rid of snails (small). only snails supply depleted, they start preying on other slow moving fish.