I agree, it's not ick, that's small, clearly delineated white dots. If it's cottony patches, it's columnaris, a bacteria that feeds only on dead tissue. I produces toxins that can kill adjacent tissue, but it's easily overcome by the immune system of a healthy fish. I've never encountered in in my tanks, even when I was breeding rainbows and selling them as a hobby.
There is no fungus that attacks living fish in aquaria, that's a myth that pet stores promote because is looks like fungus to them. Their main goal is to sell you meds and then replacement fish when the meds fail to cure the problem. Salt is the only thing you should add unless you have a sure diagnosis. Even then, the cure is likely adding more salt. BTW, salt does kill some infectious agents because they are not as tolerant of wide changes to the osmotic pressure around them as higher organisms (your fish). But the main thing the salt does is to reduce stress on the fish and promote more of a slime coat (yeah, the expensive AP products can do the same thing as cheap table salt, go figure)
In my experience, once a fish is visibly sick, they rarely recover. That outcome is the same whether you add nothing, salt, or expensive meds. I don't bother treating sick fish, I'm treating the healthy ones that remain, and treating the ecosystem in the tank to make it healthier. If the fish recovers, and sometimes they do, it's great, but mainly I want to prevent any further problems.
An ounce of prevention really is worth a pound of cure.
Now, as to the specifics here. Keep the temp about 80 and double the amount of salt. You want it well above the baseline it usually is. If you are willing to put your plants at risk, you can keep adding salt until it's very high. Seawater is about a cup per gallon, and marine salt is perfectly balanced and much safer at higher concentrations if you have any of that around. Rainbows can be slowly adapted to about half the concentration of seawater, perhaps even more, but at some point, increasing the concentration becomes more stressful than the benefit. I'd add a tablespoon per 5 gal every day, or every other day until you see improvement. When you change the water, you need to add salt at the current levels to your replacement water as well.
For water changes, be sure you are using a gravel vac and trying to really clean things. Good filtration helps too, try to make the water as pristine as possible. Good luck.
