I think the best prevention (and cure) for fish ailments is fresh, pure water. Keep up on routine sufficient volume water changes and fish will have strong immune systems and remain healthy and/or cure their own ills (after all, there's no medicine dosing in nature!).
One of the problems with comments like this one is that it assumes Ich is a "disease" or a"sickness".
This is why I was so rude earlier. Ich is NOT a sickness. Ich is not a fish disease.
***Ich is an animal, not a disease***
Ich is a parasite. It is a small animal!!
Ich is an animal that attacks your fish and eats the skin off your fish. If you have a bad animal in your aquarium, you don't perform water changes. You get the bad animal out!!!!
People have this weird problem with confusing all things that make fish sick or make fish die. They must all be "sickness". This is not sickness. This is not a virus or a bacteria. This is a very tiny little bug that is literally eating the flesh of your fish and then having hundreds of little babies.
This is also why people have such a problem using the right amount of salt to kill ich. Salt isn't a "medicine" that cures a disease.
Ich is an animal, just like your fish. Ich is more sensitive to salt than SOME fish(and only at a certain time of its lifecycle). When you add salt to an aquarium to kill ich, you are adding enough salt to hurt freshwater animals. That is the whole point. That is why it might be so much that it kills your fish!! Some freshwater animals are VERY sensitive to salt. Some don't care(mollies/guppies). You have to add enough salt to your aquarium that you start killing freshwater animals(ich).
Can ich fall off because of fish stress?
This ich=animal thing is important to the whole question of it falling off because of the fish.
Ich does whatever it wants. It doesn't care about the fish. It will gladly kill the fish. It doesn't fall off until it completes its lifecycle.
The fish has ZERO control over the ich.
Think about ticks on your own body. Will ticks fall off of your body because of your distress? No.
They might fall off if you start losing enough skin that they fall off with the skin. I don't think your fish lost enough scales to knock off the ich.