Puffers are not good fish for mixed tanks. They are obligate predators, extremely well equipped with teeth and strong jaws, and demanding on water quality along with being very messy eaters. The FW lurker predators that gonefishin mentioned (arrowhead, Congo, T. miurus, etc.) are fish-eaters (plus mollusks and shrimp), the active hunters tend more to inverts (mollusks and crustaceans) but when hungry (always, like Oscars) will give fish a try. the size range for FW puffers is from 1" to >2'