Saprolegnia on the fish is "nature's way" of cleaning up dead tissue. Often if the fish is given a salt bath the fungus and dead tissue will float off. That does give the living tissue underneath a better chance to heal.

It is very possible that Angelhologram has hit upon the issue in raising the question of uneaten food. Without a squad of pond snails in the tank, that does happen.

Around the aquarium, unfed fish food will often fungus. In fact, you can spot a pet shop which is in trouble (and they are not taking care of their charges) if they have a lot of fungusing food in several tanks. It can happen to everyone once in a great while though.

In fact, dead fish will also fungus - one of the ways of recycling the nutrients in nature.

Look at the Dec 15 entry on this site
http://www.hojleddet.dk/nyt.html

Here some angelfish eggs have been laid. A lot are good, about 50 were infertile and fungused.

I took some images of some killie eggs I had picked two days ago. There were good eggs and two (of three dozen) looked on the way out this morning. One showed a good example of an early case of fungus. The shots were taken, the suspect egg drawn out of the container and squirted into the out bucket.

It would be nice if I could get the card reader to read them! I'd find a way to show them!

At any rate, gravel vacuum around places on the gravel which are especially "blessed"  with this stuff. A lot of us have been surprised by a detritus deposit. :)

It is still possible that something else is going on, but the food issue is one of the first to look at.