I've done it, and like was said It's not recomended. I currently have one in my guppy tank and it's been there over a year. It doesn't nip my guppys at all, but I do have a lot of cover in the tank. Also, feeding is an issue, since puffers are slow ambush preditors it is possible for them to starve. I actually overfeed the guppies a little bit and the puff will eat a bit of flake, but the main reason is to help keep the snail population up. He activly hunted snails from the get go, but some individuals will need to be "tought" that the snails are food. Unless you are dilligent and experienced, as well as have a tank you could move him to if things dont work out, I would asvise against it.