Woodysun, I mean no offense or harm, and I respect your fishkeeping skills as much as anyone. Your Armatus (characin payara) is an inspiration. BUT, I just noticed you said your caribe are between 23-25 centimeters, so basically around 10 inches, and that's not too especially big for caribe in the hobby (mine is around that size also).
That said, those fish are severely overweight. And I do mean severely. I would get them on a diet as soon as possible, and add a few more powerhead jets to the tank as well. I don't think they'll get maxed out size-wise if you stay on this feeding regimen; they're simply getting more than they know what to do with, or can do with.
If you increase the workload for them (powerhead), they might be forced to swim more, and grow a bit out of those fat deposits, all without turning on each other looking for meat. I don't know what to say, it pains me to see caribe and piraya in that shape.
For example, here is a thread I started regarding my 9.5 inch caribe, which is just as long as yours:
http://www.monsterfishkeepers.com/forums/showthread.php?t=110183
Take this post for what it's worth, I'd just try to get those guys on a diet as soon as possible.