Piranha's live in large groups by nature, its how they survive, if you keep one by itself it would be miserable- for example, i'm sure you could bring up a human being in complete social issolation, but that person would live a miserable existance, and as you put it you wouldn't see the true natural/normal behavior of the individual.
There is a difference to just being alive and actually enjoying life and thriving.
The problem with keeping piranha's in crampt quarters as far as i am aware is that they have been known to cannibalise each other when they cannot cope with living with the lack of space anymore. This doesn't happen with all piranha's kept in overstocked tanks, as it mostly depends on the individual, but it does happen a lot none the less when this is so.
I would personally go with AMS's minimum amount of gallons estimate at 75gallons, it sounds a lot more reasonable for the fish involved good.gif