That is simply not true, at all. I have had both figure 8 and spotted brackish puffers. I kept both kinds for years in my brackish tanks, far before my saltwater addiction. They were over four years old when I finally sold them to a friend, so i watched them through every stage of their life, and every habit change. Brackish puffers dont get much larger than a few inches around, and if anything, they are scared of everything else in the tank. Mine never bothered anything at all. They even ran away from the bumblebee gobies, which if youve ever seen, are probably the smallest fish you could find (think one of the small yellow clown gobies for SW). Brackish puffers are NOT AGGRESSIVE and never bothered gobies, scats, eels, archers, monos, or anything else, smaller or larger, in my tank. The only thing they are aggressive towards is snails and small inverts. So the correct information is the exact opposite as above, they will not attack anything in the tank. And they love tankmates, they have incredible personality and the only thing you should worry about is other tankmates attacking the puffer.
They do not have a high mortality rate either. On the contrary, brackish fish are born in fresh and brackish waters, live their young lives in brackish, and move into full marine later in life. So they are genetically set-up to make the transition quicker than you'd think. In the wild they don't move a few feet a year, it's actually a quick transition. When I first purchased my eel he was in freshwater, where he lived for six months. I then transitioned him into brackish water in a matter of hours, no ill effects, actually he ate heartier and quicker than any other acclimation I've seen. When I converted him to saltwater, I did it over the course of a day since I was busy doing other tank maintenance, and again, he was fine. The only fish I took a while to convert was a 4-yr old green scat, since he was sick at the time, and I did it over the course of a week, doing 10% changes every few hours or so. They are actually easier to convert than it is to acclimate new fish. It's a simple process, and it's one their bodies are used to going through anyways.
As far as corals go, I have not attempted this yet, but my brackish puffers did pick at a plastic bag once while I was floating a bag w/ some snails in it. So if youre feeding some fw pest snails, and theyre on coral, it'll be munched. And the puffers might pick at the corals as well at first, but I haven't heard much about them and corals. Scats however (esp green scats), are EXCELLENT for removed bad algae blooms in SW tanks, and ive had several people ask me where they can purchase green scats exaclty for that reason.
Good luck, lemme know how the transition and corals go, i'm interested in learning as I've still got a full tank of former-brackish fish now in full salt, and am curious what else I could add.
