I feed LRS Reef Frenzy, Herbivore Frenzy, Mid Jersey Angel Diet and Reef Pro, Live white worms, Frozen, mysis shrimp plus stuff for the eels like squid and silversides.
I think it is the entire package though. Biodiversity in the tank, flow and varied feeding so a fish doesn't become nutritionally deficient in any way. Effective filtration that maintains excellent water quality. I am not talking about particulates you can catch in a filter sock, I don't even use those.
Make your tank as much like the sea as you can. It can never be a perfect copy. But if you get close enough I think a fish can stay healthy and shrug off the challenges it would normally be able to cope with in the ocean.
I strongly suspect that ich isn't a powerful swimmer. Decent flow may be able to disrupt it's life cycle by making it difficult to find fish in large numbers and return to the bottom to encyst. I do not turn it down at night either. `I also suspect the normal microfauna of a tank has some members that eat the cycts they find since almost everything in the sea is food for something.
A human placed in a small sterile space stale air and a monotonous diet would not thrive. Neither will a fish.