im positive. they hit a growth spurt maybe around 6-8" and will grow so quickly, you can literally see the difference one day to the next. they generally seem to stop between 16 and 20" or so. and yes, this has happened to 4 of the 5 pacu ive raised recently from juvies.
i got one of my pacu in the start of 2007, and he was the size of a silver dollar coin. he's now maybe an inch or so shy of 24", and has been growing extremely slowly for the last year or so.