the 18inches is whats going to cause problems, considering these fish top out at around 18+ inches... i think thats the main reason for a 125, being that it has a front to back length of 24inches...
So in other words, i would seriously try and get something with at least 24 inches front to back, and no less than about 4 -5ft long... otherwise your puff may have a hard time turning around as it gets older causing problems...