There is absolutely no reason a fish in captivity shouldn't exceed the size & longevity of it's wild counterparts, if fed & housed properly. I have never kept a T mbu, because I could never house one properly. I do go by the advise given by the Mentor at my forum, who has been keeping puffers for over 50 years & is a research scientist, author & lecturer in the field.
"For me the absolute minimum would be 10X4x4', and I would want significantly larger myself, a multiple of that, if I ever kept another T. mbu - which is highly unlikely. A two-foot high mass fish is not a trivial bioload."
"IMNSOHO (In my not so humble opinion) that whole concept is mythology. QT tanks are special cases, they need only fit the fish for a few months at the outside. To me, a fish should move from QT to its permanent home. If you cannot set for that immediately, you should select another fish. Many of my fish, and all the "special" ones, move directly into their permanent home even for QT.
"I still consider a 1000 the absolute minimum for one T. mbu, and personally would not keep another without something significantly larger than than that."
"My mbu tank was too small. I compensated in part by massive water changes, 1/3 every 2-3 days (twice one week, three times the next). The fish had permanent damage to his caudal fin from too small a tank in the "moving up" process. I will never again do that to any fish, or strap myself to that level of other-than-automated changes."
"Also IMHO, your tank is way too small for any T.mbu - even a juvenile. They need to grow fast. The common practice of "moving up in tank size when it is needed" is IMHO excellent advice for stunting fish. Big fish need big tanks, swimming/exercise space when they are small/juvenile. I learned that many years ago with Cichilds, and finally caught on that it applied to puffers and most other medium to large fish equally. If you want normal development and configuration, and normal adult size,and long life, you really need to provide"permanent" homes from the start."
"Big predators do not need tankmates. Appropriately sized and equipped tanks for these fish is barely possible in the home. Tankmates might be suitable if you have a greenhouse enclosed swimming pool sized 'tank' for them, otherwise you likely are doing the fish a disservice."
"IMHO & IME, T. mbu should be in a massive tank from day one, in FW which may hard or soft, alkaline or acid, but moderately alkaline is the easiest to manage with large predatory fish. They should have no tankmates, period. IME, Tankmates of any size will will be food sooner or later. They should never under any circumstances be co-housed with F-8 puffers F-8s are BW fish, T. mbu is FW. Do these folks keeping mbus forget that they do not have to swallow their prey? They can take chunk out of any fish - they may go for easier, smaller prey if available, but will not hesitate to "taste" a larger fish if hunger strikes. Being nocturnal is no defense. They are massive fish and should be kept in massive tanks. I don't like to offend anyone needlessly, but keeping a T. mbu in ordinary tanks such as 120 and 180s is serious and harmful unde-rhousing - exactly comparable to keeping a GSP in a five-gallon tank. If you cannot plan on 1000+ gallons, with massive support equipment to match, do not consider this fish. I would never keep another in less than multiple thousands of gallons. An 8 x14' room on concrete was inadequate for keeping my T. mbu. Been there, done that, and would dearly love to have another - but I know that I cannot properly accommodate the the fish's requirements. so I will not ever get another - even though it was my favorite fish of all time. Mbu was a member of the family for over 12 years, and will never be forgotten.
All IME & IMHO, but I do sincerely believe that if you cannot do it right, you should not do it at all. My mbu had a permanently deformed caudal from being housed alone in a 180 - way too small, even if he did reach full size."
"I think that 300 gal is still too small for good water quality with an adult mbu. I would guestimate 8'x3'x3' as minimum tank size for the fish itself, plus some large external filters - say a couple of 180 gal, veggie filters on opposite light cycles plus some massive mechanical filters and an automated water change system."
"To those who doubt the minimum size I suggested, I can only ask: How long have you kept a T. mbu, and to what size, in how much water? We are always open to other's real-world experiences, but some cranky old men have very little patience with guesstimates not backed by experience. Do you have any idea of the mass of a two-foot puffer?"
"IMHO & IME, any giant puffer should have no, repeat no, tankmates. Tankmates are called: sushi. T. mbu are massive fish and need huge tanks and support equipment. Figure on 2 feet minimum, ... and housing, 15 -20+ years. "