For your tank you can do either 2-50% and one full water change a wk OR you can do what i do . I do one full water change every 5 days.
Make sure you acclimate him when you do a full water change(the new water temperature has to approximately match his water temperature). Let us know if you need any instructions on that.
Do you think he still has fin rot?"smudged" , darkened , ragged or frayed edges, fins falling apart in chunks-this indicates fin rot.
If they are the same color as his body, or have white/clear tips, this indicates normal growth
If he still has fin rot i would do more water changes and if you have stress coat , it would help with healing. Did you treat him with medications or salt?
Good nutritional food will help with recovery . What kind of food you feeding him?
Also frozen blood worms are really good source of the protein and good for an immune system.
Bloodworms - Cubes - 3.5 oz. - Frozen | thatpetplace.com