Geez, another one.... I sure hate reading about this over and over....
If it is ich (sounds like it from your description), and you decide to QT, go the hyposalinty route, instead of chemicals that may do more harm than good. And hopefully you have a fully cycled canister that is available to prevent ammonia.
With chemicals (other than copper, which is bad for other reasons), you get false hope, as it recedes temporarily, and then comes back worse, often in the gills, and your fish will die quickly. With hypo, the 2nd leg, if you get one is milder, and then you are home free after that. The fish are much happier in hypo.
I've done both in the past, so not saying this from reading on the internet. I now QT religiously before adding new fish.
Best of luck.
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