Arthur, the unexpected fish I got from a pet store on Sunday, was very sick for a few days. He barely came to the surface for air, and I was crossing my fingers for him to pull through and bracing myself to euthanize him.
And then, a MIRACLE! I upped his dose of AS at his water change (1 tsp from half a tsp) and the turnaround was incredible.
It was like he was a new fish!
His fin rot is almost gone as well, after three days, and his swimming isn't jerky like it used to be. I'm so happy and relieved for him! I'm glad that he pulled through; one of the reasons I felt drawn to him was that he reminded me of my (late) Merlin. THANK YOU SO MUCH to OFL, Bettaluver4evr and denaliwind for their concern and advice.