My son had a puffer fish in a freshwater tank. A 15 gal. one species only tank. We put some guppies in there to keep him company.l He was so-o-o-o-o cute. He came up to the top every day like he enjoyed company. We started him out with fresh brine, and he would chase them around snatching them up in his little beak. They nip the fins of other fish so observe closely if housing with other fish. Recommened with larger tankmates. They say you can adapt them from fresh to salt water by adding a little marine salt at a time. Eventually we put him in a community tank, and he did not do well.He got stressed, attacked and inflicted with an open wound which he died from.
I recommend not getting a porcupine fish, unless you are keeping him alone as he will nip at bottom dwelling fish, slow-moving, and long-finned fish.
c-c