My favorite is people who suggest heat AND salt, and when confronted to explain why both are needed they miraculously go silent (not so much the case in this topic, though). High temp kills Ich. Salt kills Ich. If both are effective, why the need for both? Why not throw in some malachite green, too? See where I'm going with this?
There are three stages to the ich parasite. First where it becomes attached to the fish (the visible stage, and the first of the three). Second being when it detaches from the fish and falls, and third being after it has landed and is on the ground and has become free swimming. Basically (and this has been from my understanding, which could be incorrect) heat speeds up the life cycle of the parasite, making it grow at a faster rate and detach at a faster rate, but once it gets to a temp of 86 F or higher, it can no longer survive. Kinda like how a person can't survive in a temp of more than 104F for too long.
Salt, like I said already, disrupts the osmotic balance of the parasite, effectively drying it up through osmosis. Using a hypertonic solution (with salt) will cause the water in the cytoplasm of the parasite to exit, and then dry it up.
So, with heat and salt, you get something effective. Heat speeds up the lifecycle, making the parasite fill and detach faster from the fish(or if the temp is high enough, this being the preferred way, the parasite will die), and the salt dry the parasite up.
Medication is only effective in the free swimming stages of the parasite, as this is when it isn't protected by it's cystic membrane.
Generally speaking, you could kill ich with just either heat, or salt, but both is more effective and more preffered the medications.
Again, this is my attempt to explain, using some bio knowledge. I could be wrong on some of it.
