Initially I though it was minor damage that would heal over the course of time, however in the past week it has becme more noticable and does not appear to be healing. It looks like the fin has been eaten away. The fish behaves normally and no other fish in the tank have any similar symptoms and there have been no desease outbreaks.
pH is low, about 6.2 - 6.4 however nitrite is 0 and regular water changes are carried out. kH is 1 and gH is 5 as measured with sera tet kit (approx 90ppm)
What would you recommend to treat it? I have melafix on hand, should I dose the tank with it or try to net the fish out and apply it directly to the affected area? Any help would be appreciated as I do not want to see the fish deteriorate any further.