In my opinion, the lack of growth seen in captive rhombeus piranha is due to the keeper.. take for instance one of our members, Joedizzle had grown his rhoms out more then 2 inches in about a month.. One thing to take into account is most wild rhoms are caught with some type of intestinal parasite. in the wild these fish are eating constantly and it probably doesnt hinder the growth of these fish much.. in captivity, on the other hand, most piranha keepers tend to feed their piranha just enough to keep them hungry. Maybe not on purpose but through my research i find piranha, especially those from the serrasalmus genus like to eat less more often, rather then alot all at once.. another thing to take into account is the fact that the fish roam constantly and are eating all sorts of diverse foods. Including but not limited to whole fish, fish fins, scales, nuts, and fruit. Which most keepers cannot replicate in the aquarium. One more thing that comes into play here is the fact of space and water quality. In the wild if a fish encounters a toxin in the water they can just swim away and not have to deal with any reprocussions, but in captivity the fish cannot swim out of his little glass box and is relying on us to keep their water in pristine condition..
JMO though, you dont have to listen to me