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Can I treat this fish alone in a slat bath for ich? If so what is good salt to water ratio?

Can I treat this fish alone in a slat bath for ich? If so what is good salt to water ratio?

Treating the fish singly will not help so much with this infestation and the extra handling will stress the fish even more.
The tank needs to be treated.

The adult parasite burrows into the skin of its victim, feeding on blood and dead epithelial cells. The irritation caused by the burrowing parasite causes the skin of the fish to swell and produce white cysts seen as a small spots.
It's not unusual to see infected fish scratching against rocks and gravel in an effort to get relief.
After several days of feasting, the engorged parasite develops into a trophozoite, burrows out of the fish and sinks bottom of the tank. Secreting a soft jellylike substance, it forms a protective membrane inside of which it divides into hundreds of baby parasites, known as tomites. The hungry tomites soon leave their home in search of a fresh fish to dine upon.
It is during the free-swimming stage, which lasts a mere three days, that the parasite is vulnerable to medication. Once it has burrowed into a new host fish it is safely protected from chemicals in the water.

Good info from Birger. I would not do the bath either. If, for some reason, you are adamant about trying to kill the parasite while it is burrowed in the skin, salt may not be enough. I would think something like potassium permanganate is needed, a strong oxidizing/anticeptic agent used in septic system industry. It will, however, burn the fish's gills and one must be very careful. I can look up the advisable concentration of KMnO4 but, again, I would not do it - it is hard to kill the parasites and not kill your fish. One could try applying the permanganate solution on the body while holding fish out of the water for short amount of time, which, of course, is even more stressful but I think, still, less probable to kill your fish.

Needless to say, the ich is in your tank. The whole tank treatment is needed. Treating a particular fish out of that tank, as you are thinking, may be advisable if the fish is the worst case and not recovering or, say, is apparently dying from the parasitic infestation that is already on it.