Sorry, R.I.P., I wouldn't buy anymore fish to you find the problem out, if it's poor water quality or gill flukes, as you are just wasting your money, get water tested if that ok, you could be dealing with gill flukes.
http
/www.fishdoc.co.uk/disease/flukes.htm
Hope it isn't gill flukes little ******** to get rid of, i've had them in my tank, had to strip it down in the end, and medicate again, but now they have gone, here is some more info for you but i'm not the writer of it.
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.