First, piranas cannot be "shoaled" in the literal sense, shoaled makes it sound like they are going to be peaceful like neon tetras or guppy's and nothing could be farther from the truth. So we come to compatibility. Most p's are not compatible because they engage in finbiting and sometimes mortal bites. This behavior is most evident with Serrasalmus species. Because some species like S. maculatus are found in small groups in the wild, it seems some hobbyists think their fish tank is the rio Guapore. It's not. Its an unnatural situation and they are confined to a small space. What does a photo op prove? nothing, except to those gullible to believe its worth a million words. Realistically, Serrasalmus species when juveniles can exist with a minimal of damage (loss of fins or eyes) for a short duration.azeral26 Posted Today, 03:43 PM
I was reading on Shark Aquarium that these species can be shoaled (Serrasalmus medinei or sanchezi). Anyone tried it?
What constitutes a short duration? Anything from a few hours or until they reach subadult size. What happens when they are grouped together basically depends on several things, water temperature and tank size are considerations. Piranas are unpredicable and for anyone to suggest they "shoal" is either irresponsible or lack any real knowledge of the beast and what the term "shoal" actually means. Science (W.L. Fink, 1997) clearly stated they do not shoal in the literal sense. And that is the world foremost authority on these critters. I would certainly take his observations over some guy on the internet.
So does that mean you shouldn't try? As a collector, I wouldn't simply because I enjoy my fishes pristine and undamaged. So it comes down to what you are after. If it is grouping of the species then mortality or fin damage are something you'd better seriously consider. Don't be fooled into believing a photo taken at a moment of time to suggest it is long term "shoaling". It's not worth anymore than photos of BIGFOOT suggesting the species is real.