I don't think your parameters are "all at good levels" considering all you do is a 50% water change once a week.
I think he has fin rot.
In a 2.5 gal you need to be doing one 50% and one 100% water change one a week.
To help heal his fins, you can use API Aquarium Salt for 7-10 days. The dose is 1 tsp./ per gal. So you would use 2.5 tsp.- with 100% water changed DAILY and replacing the salt after each change for up to 10 days.
This will help with fin re-growth. It's also best to let the salt dilute into a cup of warm water before adding it.