What is your quarantine plan? Goldfish coming from ponds do harbor a lot of parasites including flukes, anchorworms and fish lice. I had mixed infestations of those with my own goldfish and that makes everything doubly frustrating for me.
Dimilin or trichlorfon is the only way to get rid of fish lice. Adult fish lice will need a higher concentration of either meds to kill them but I generally pluck off the adults and dose per regular instructions to focus the concentration on the juvenile ones instead.
On the other hand, I'd advise against using goldfish for your piranhas. There are far healthier options.