pacu teeth basically look like your two top front teeth. their rectangular/square shaped, and quite sharp. these guys naturally eat tree nuts that fall into the water, and also the occasional snail, so their teeth are designed to crack them easily.
my biggest guy used to bash into the glass tank when he was in there, and had his teeth actually exposed from losing the flesh around his nose. got a real good look at those teeth for awhile.
to the OP. hope your ok, besides a little startled and cut up. could have been worse, looking on the brighter side. he could have taken a real nasty chunk out of there.