I’ve been eating and catching fish for over 70 years in the U.S. and have yet to see one of them infested with worms or parasites. Sounds like an urban legend. What country do you come from?
On Vancouver Island I caught a bunch of trout that were wormy. The 7 acre pond they were in was pretty warm, so I think that had a LOT to do with the problem.
The fish were also very logy, you could actually bounce a lure off their head and they would ignore it for a time.
Filet an old fish, plate the filet, salt the filet, stick the raw salted filet in the fridge. Check back in 15-20 minutes. The worms will rise up out of the filet and wiggle for you.