according to AMS (piranha expert) the red bellys would run away from your arm, as long as you aren't bleeding, so, they aren't going to kill you lol
I agree with demonmagus, they will more than likely hide away just like any other fish would, just make sure you arent bleeding
My Gourami will touch my arm with his feelers and nibble at my fingers the entire time my hand is in the tank.