I don't know being a enthusastic koi karp person myself (also some goldfish, golden orf and a few shubunkins) I'm not sure if that person would be able not to notice the difference.

Thus, my solution would be that I would hypnotise them into believing they never had a goldfish, then give them a goldfish!!! Instead of being a bad friend you're a good friend!!! Now am I good or am I good??!!!!!!

OK.....would you ever fly to the moon......to destroy it? (you Xindi collaborator!!!!!!)