Ime With ich you will generally see spots before they hit the lethargic stage. What kind of stuff was growing on him? You may have treated for the wrong thing, a lot of times its not the disease that kills the fish, but the treatment instead. Have you changed your filter media recently? Anything you've done different recently? Sorry for the 21 questions but its hard to say what could be going on without those test readings. Does this pleco have white spots?
However if your cycle crashed somehow you would probably see symptoms in your angel before anyone else
How old is the tank?