Why not make your piranha bite you? That was a dumb thing to do. Not to mention, it was probably stressful for the fish. You squeezed it so it would bite you, and held it in the net like that. Both would generally be considered no-no's.
Also, if it's being kept in the 55, it's not in adequate conditions. Just saying. It's either overstocked or lonely. (minimum school would be at least 3) 1 piranha per 25g is a general rule. Do a little math (might not be the easiest thing for you), and you would need a 75g minimum aquarium.