I kept 9 in a 29 gallon and didnt have any aggression issues, but it was a species only tank. I have always heard they were notorious fin nippers, and have seem them doing it to larger fish at the store before i bought them (was the reason i went species only) was pretty cool when it came near feeding time, they would all swarm together and from a few feet away it looked like a bunch of bees hovering in place. Cant speak for anything long term, only had the tank setup for about a year, but didnt loose any of the puffers before i sold the tank.