Great news! He's officially been swimming normally for 24 hours! I've been doing 90% daily water changes and adding salt each time. I had to order maracyn 2 because I couldn't find it in store. It got here late yesterday and i didn't have any time to do anything because of work. So I'll hold on to it for now in case things don't keep getting better. But his improvement is amazing over the past 24 hours.
The plan is to keep him in the smaller pond until I figure out what happened in the bigger one. I've got some minnows in there now and I'm monitoring their health to see if there's still a contaminant or what. Fingers crossed whatever it was is gone