Takifugu puffers are aggressive to alot of other fish, I had mine in a freshwater for a while with a TSN and black ghost with no problems for a while, if I introduced a new fish, the puffer would attack it. They do have to go into brackish eventually with a higher ph. Usually within a year old. Some say you can keep scats or monos with them because they are fast enough to get away but I suggest to keep them alone.