Most medicines are rather harmful, to almost all life. The hope of the medication is to stress the lower life forms, like the protozoa ich, and kill them before the medication kills the higher life forms. This is why correct usage and dosing are important. You want enough concentration to kill the protozoa without so much that it kills the fish.
This is true not just for fish medication, but a lot of human medication, too. A lot of them are poison to humans, but we don't take them long enough or at a high enough concentration to kill us, but hopefully long and strong enough to kill the infection or issues the medicine is to fix. Chemotherapy for cancer is probably among the more dramatic examples. It is always a delicate balance to kill the cancer cells with the very dangerous chemicals, without killing the patient. There are a lot of chemicals that kill cancer cells out there, but not a lot that don't also harm the person.
Further complicating the issue is that each species has a different tolerance, and even each individual will have a different tolerance. We all know that on person who catches every cold, and that other person who never seems to catch any colds -- fish are similar. Each individual will be a little different.
Regarding ich specifically, please take a look at this link for a lot of good info: http
/webcache.googleusercontent.com/search?q=cache:nB4zJTXJ2tIJ:www.skepticalaquarist.com/docs/health/ich.shtml+internet+archive+skeptical+aquarist+ich&cd=1&hl=en&ct=clnk&gl=us&source=www.google.com
The actual site seems to be down at the moment, but fortunately, Google seems to have a cached copy.
It will talk about a lot of the myths and misconceptions about treating ich.
In particular, once an individual has it in your tank, you need to treat the whole tank. Ich has a swimming life-stage so those swimmers are still in the tank ready to infect more fish. If you only take the ones out that show spots, the other fish will also be attacked and eventually show symptoms too. So you want to treat the whole tank for many days after you see all symptoms disappear. Otherwise, you'll be back here complaining about the ich that will never die.
