Don’t use Melafix at any dose with Bettas. If it continues, I have had great success with giving Epsom salt baths (1 tsp to a gallon of water) 3 times a day. Keep fish in bath for 2 minutes each time. This helps pull fluid off the eye and makes fish more comfortable. I also treat with Kanaplex and Furan-2 as directed on package. Don’t let it go too long as it can cause blindness. Good luck!