How to treat Betta fish diseases

[zephyr8]

What's your water change routine? Even with good params, old water can have dissolved organics that stress fish out. I do 20% twice a week for my bettas.

[onyxfin]

Could be internal parasites too. If he's not eating, try soaking his food in garlic juice. It's a natural appetite stimulant and has anti-parasitic properties.