Hi Anna-Maria. I'm sorry I wasn't able to get to your note in a timely manner, but I've been quite busy this past month.
As for the black growth in the goldfish's gills, it is hard to say once you find the corpse. The reason is that it is possible that the damage may have been done post-mortem (after they died), or that the fish, when it was dying, sunk to the bottom and just sucked in extra sludge. I can't say for certain that it was a disease unless the growth is seen on a living fish. There are some diseases which specifically attack the gills, such as gill flukes and certain types of fungus. If it was indeed a gill disease that got him, then the black growth sounds most characteristic of a fungal form of gill rot, which usually attacks fish that are already somewhat sickly (immunosuppressed). Have any other fish died since then?
