As per our discussions in chat, I don't believe this is a case of fin rot. First off, the fact that it did not respond to clean water or melafix was a big indicator that something else was wrong. Additionally, from the pictures provided, it seems the fish was hardly very active either, as it is bottom sitting in all four of your pictures. Furthermore, finrot does not eat only the membrane and leave the rays, but rather disintegrates the whole fin, which causes an eaten away look rather than a frayed look. Both costia and hexamita can cause fin fraying in their later stages, and considering the speed at which your fish expired, I'd be more suspicious of one of them as the culprit. I hate to say it, but if I were you, I'd watch the rest of your stock very carefully.