Although it's not Fin Rot, I suggest you move him back to the 1/2 gallon, and fill it all the way with warm dechlorinated water. What you can do to help heal his fins is to add 1/2 teaspoon of Aquarium salt to the 1/2 gallon twice a day for 5 days. His fins may not have completely healed up by the fifth day, but they will show some improvement. However, if they seem to not be showing any improvement or they get worse, continue treatment for 3 more days.
If you can, I would also do a large water change (80-100% depending on the last water change) on the main tank.
He really is a beautiful fish, I wish you two luck!!!
- Dead Sunlight