I would assume that something like fin rot would spread to others in the tank.
My hypothesis, since you say you got the fish in this condition, would be that it was some sort of netting injury when the fish was caught in the lake. Then was shipped to you without having a chance to recover - that kind of stress isn't conducive to healing.
I'm not sure what the ulcer-looking part of the injury is, but I would isolate the fish and treat outside of the main tank, especially since you say no one else is affected. Keep up with the salt and lots of water changes in the QT and see if it doesn't start to heal. If it still continues to get worse, then try something more intensive.
My hypothesis, since you say you got the fish in this condition, would be that it was some sort of netting injury when the fish was caught in the lake. Then was shipped to you without having a chance to recover - that kind of stress isn't conducive to healing.
I'm not sure what the ulcer-looking part of the injury is, but I would isolate the fish and treat outside of the main tank, especially since you say no one else is affected. Keep up with the salt and lots of water changes in the QT and see if it doesn't start to heal. If it still continues to get worse, then try something more intensive.
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