Don't give them food for a few days, and then try again... Once hungry, piranha's are more likley to accept new food sources.
On a sidenote:
Don't post ipod-links here again: it's not appreciated, let alone a decent thing to do - this time I edited your post, the next time it'll be deleted and your account suspended.
Thanks.
A forum community dedicated to Piranha owners and enthusiasts. Come join the discussion about breeding, health, behavior, housing, care, classifieds, and more!