Am I correct in thinking once a fish has visual white spots there is no healing that fish or destroying the parasite, and that only the ones that can be killed are the ones not latched onto a fish? So if a fish has spots it has a very near death? At what stage would large water changes and temperature increases not be enough before you have to use medicine chemicals?
Ich is a parasite which has a life cycle that lasts approximately one week, at normal aquarium temperatures. In warmer water the cycle will be quicker (I'm not sure just how many days less, doesn't matter). At a temperature of 86F/30C it will basically be killed. However, in stubborn cases some may slip through; at 90F/32C this is pretty much game over for the parasite.
The spots you see on the fish are the second phase, and there is nothing that will get at the ich when it is protected. However, this does not normally kill the fish. I have had fish with one or two visible spots and these have disappeared and no more occurred and the fish basically fought it off. As the spots drop off they fall to the substrate and rupture, releasing innumerable parasites that must find a host fish within 24 hours or die; it is at this free swimming stage that we can kill them via the heat, or treatments. Removing the fish from the tank for two weeks will obviously kill off the ich, but the fish have to be treated anyway so it is less stressful to leave them in the tank and raise the temperature.
In very stubborn cases, and when the highest temperature may not be safe for the fish species, salt is without question the safest additive. Most fish can tolerate salt before they can tolerate any of the so-called "treatments." These may not be effective some of the time, and they certainly will kill fish some of the time, depending which ones. Salt is far safer, if something beyond the increased temperature is deemed necessary.
Fish will build up a resistance or immunity of sorts to ich, if they are healthy and if they are free of stress; if this were not true, by now there would be no fish left in nature, as they deal with ich there. In fact, it is strtess and only stress that causes fish to succumb to ich. I have many times acquired new fish and while in quarantine observed a flash here or there, a sign of ich which first attacks fish in the gills where we cannot see the spots, but I do nothing and this ceases within a few days at most. Stress is the direct cause of 95% of all fish disease in aquaria; the various pathogens may be present, but it is only when stress has weakened the fish that they succumb. This is why preventing stress by providing what the fish "expects" in temrs of its environment is so crucial to healthy fish.
Which takes me back to your question about killing the fish...only if the fish is under severe stress, and the ich has advanced well beyond the first week or so, will fish die from ich. But at this stage, ironically, they are just as likely to be killed by our "treatments."