Its hard to tell by the pic-if it is a normal color change or if its an infection of some type.
What I would do-QT and start aquarium salt 3tsp/gal and tannins- with a low normal temp and good nutrition.
The small temporary containers some Betta are sold in work great for a QT-but anything will work that is small enough to float in the heated tank to maintain temp of 75-76F for treatment or if you have a 1-2gal that can be heated to use for QT-Be sure and attach it to the side of the tank so it doesn't sink.
Premix the treatment water in a 1gal jug of dechlorinated water-Add Aquarium salt 3tsp/gal and a tannin source if you have one-Either IAL (1lrg crushed/gal
) or naturally dried and fallen from the tree Oak leaf (20 crushed/gal
). Let this steep for 30min. Shake well before use....
Using the premixed treatment water make 25% water changes every 15min for 1 hour today.
He needs to stay in the QT-in the treatment water for the duration of the treatment period.
Tomorrow-using the premixed treatment water make 50% water changes daily for the next 9 days to complete the treatment.
If you used a tannin source-the water should look darker each day and this is what you want-don't worry about any leaf bits that get into the QT with water changes.
Nutrition-good quality varied diet fed in small frequent meals-If you have access to live mosquito larva-offer several rinsed a couple of times a day to boost protein intake to support the immune response.
Keep us posted....