Piranha, while cool and vicious looking with a reputation that is the tale of legend's,
in captivity they are rather wimpy, nervous and an overall boring fish, IMHO.
While it is true that a school can strip an animal rather quickly, if you looked closer it's a lot of skittish fish doing "hit & run" attacks. So while hundreds of them in a school is awesome horror movie fare, 8 in an aquarium is
"meh".
as mentioned above, they need to be in a large biome, not a fish tank. for them to be at their best and most natural you need to recreate an Amazonian type tributary habitat for them.
That is something most folks don't/can't do, so rather than have the fish hide in plants and décor in a small tank, they go with a bare tank, and the fish live out their lives stressed out as if they drank a 12 pack of red bull.
and there always is that thing about them hitting fast and taking chunks out of your hand if your not careful...
They really are another fish that should ideally only be kept by the most ardent and experienced collectors, IMO.
and definitely don't get pacu, unless you like ugly fish.
If you want gnarly mean fish go saltwater and get triggers and moray eels, you'll forget all about piranha.