Join Date: Oct 2019
Location: Johnson City, TN
It looks like physical damage to the mucous coat. I've seen skin damage like this from suckermouth catfish, kissing gouramis, etc. Salt addition of 1 tsp/gallon will help with osmoregulation & mucous repair, regardless of your other treatment(s). That amount has little effect on the plants I see (Java moss, Java fern, Hygrophila & Anubias?). I'd change 25% of the water daily at a minimum during any treatment, especially if using antibiotics.
Are your nitrates something you can test? Also, the angel is showing some signs of aging decline in the loss of fleshy fat reserves in the head, making the eyes appear more prominent. I wish you well in bringing this fish back to health.