My only experience with a potential "monster fish" was with the black ghost knife that I inherited when I assumed responsibility for an abandoned 29-gallon cesspool. Finley was about 6 - 7" long and had been in that tank along with some guppies for about a year and a half. After I took over the tank, I had it professionally cleaned (that alone took two months and a minor fortune to fully do -- believe me this tank was like primordial soup) and totally redone with proper substrate and real plants. I also began to feed a varied diet. I kept Finley for about six months more and he really began to grow. I realized he needed to go to a much larger tank, which I couldn't provide at the time, so I had no choice but to rehome him with a friend who had a 55-gallon. He adjusted immediately to his new home and lived for another year or so. Don't know why he died; my friend woke up one morning and he was gone. So, for myself I can say Rach is right. BUT I also know that, FOR ME, I don't want to buy a fish that I cannot "properly" keep longterm. So, if I don't have the room/money/energy/whatever to keep and maintain a properly sized tank that will ultimately be needed for my pet (I think of my fish as my pets), I have no business getting that fish. I love black ghost knifes and Finley was the best, but I inherited him. I'd love to get another, but right now and for the forseeable future, I'm in no position to have the size tank I would ultimately need for such a fish, so I won't get another black ghost knife or any other "monster" fish.