All nems can eat fish, but the carpets do have a rep for being fish eaters.
What alot of people do is get clowns, and get a nem that isn't a natural host to that species of clown, and success on clown nem hosting can vary w/ that.
At top of this forum is a sticky that has nem facts, which shows which are natural matches.
My personal preference happens to be sebae and LTA, but your perc may or may not take to one of those as a host.