Sick Cardinal :(

[GoFish]

I first noticed some skin blemishes on one of my Cardinals about a month ago. No experience in ID'ing or dealing with sick fish if anyone can ID that would be helpful. It looks like the flesh is soft and rotting in areas, the left side has damage worse than the right

It eats and acts normal, good appetite, its mate is healthy and shows no signs of problems as well as 2 Clowns they share the tank with

Click on image for video...