This does indeed drop off of a razor fin piranha; one just dropped for me today.
There are two areas where piranhas spawn, so make sure the clear them both out.
Greenblood piranhas and large greenblood piranhas are their placeholders and I believe they have a 20min or so spawn time.
Good Luck!
Edited, Mar 2nd 2010 11:10am by Ebelch