I have a gold fish that has been in my pond for a few months and it has developed black smudge disease and an ulcer on its side.
I have always been under the impression that this condition develops when the ammonia level is too high.
My test kit shows PH around 6.5 and ammonia around .5 ppm as and the color chart shows the water in a safe level.
Non of the other fish seem to be suffering this condition so my question is: what else can cause black smudge disease in goldfish?