If you haven't added fish in the last 72 days, and your fish are all asymptomatic, likelihood is that you don't have ich in your system. The longer you go without adding a fish, the likelihood rises. Ich is an animal. It has a very brief life cycle, and it dies in the act of reproducing or trying to reproduce. If there is a success, there is more ich, but if there is a failure, that particular ich just dies, and that's the end of that particular animal. There is, to my knowledge, no do-over for a failed ich on a reproductive bent: it's already going to die, win or lose.
In your situation, if you truly can't withdraw fish to treat, the only thing to do is feed well, use something like Selcon (vitamins and omega-3) and heck, feed them fresh chopped garlic. Does NOT cure ich. But I've known vegetarian fishes to like it and go after it even if they're rejecting Nori, and it stimulates appetite.
Ultimately, if you do lose all fish, you'll have saved your tank. 72 days after last appearance of ich on a fish in the tank, it's likely ich-free, and you can start from there by putting a fish into a quarantine/antiparasite procedure.