Well water quality is good you could have gill flukes.
Not the writer of this information below.
Gill Flukes
Symptoms:
Fish with gill flukes have gills that may appear red and inflamed, bleeding or slimy. Excess mucous or puss may exude from the gills. Sick fish can be observed gasping for air near the surface. This disease is most common in younger fish and fry, who are more susceptible to the parasite.
Cause:
The monogenetic fluke Dactylogyrus, which destroys gill tissue and damages blood vessels in this region.
Treatment:
Treat with Clout, Fluke Tabs, Paraform, Trifon, Paragon, Quick Cure, Formalin, orParasite Guard. Gill flukes are highly contagious, therefore, all fish in the same aquarium should be treated.