Thank you for responding!
From the onset of symptoms, I would say it took a month or two for both fish to die. No reddening of the gills, but yes, body rapid breathing and laying on the substrate. Water parameters with the second fish were Ammonia: 0, Nitrates: 0, Nitrite: 0. The first fish was Ammonia: .5, Nitrates: 10, Nitrite: .5 when I got home from vacation, I got the ammonia and nitrite under control shortly after I got back.
I followed this protocol with the second fish: https://www.aquariumcoop.com/blogs/faqs/how-to-use-quaratine-med-trio. With the first fish, I did try some sort of medication in the tank, but I can't remember what it was.