If it's not sand, then I'd bet on flukes. You can try PraziPro like ppl mentioned above. If it turns out to be ich, you have very straightforward answers: Treat them with copper or hypo or DONT treat them and live with it, if your fish is healthy, they probably won't die from ich.
By the way, you better change your mindset if you think of killing a fish cause it's sick. They're living animals that you've chosen to take care of, so do it!
Good luck!