Update: Its my third day doing 50% water changes. So far today, he seems far more lively. He won't let me "handle" him, that is he won't rest on my hand under the water's surface where I can get a good look at him, whereas when I first asked for help he had no issues being handled. This to me is a good sign. He's swimming well and fairly active and has no problems floating or hanging out on the bottom. His eye even looks slightly better, though I haven't had a good chance to check out his cloaca today.

I'm hopeful and appreciative of all your help. I'll update again in a few days.

Also I'm using the Seachem Prime. Its a great product.