There is a condition where a snail picks up parasitic cercae, eaten, by and passed on to fish, which when eaten by a bird, are passed on when bird droppings end up in the water, and the cercae start the cycle over, and over.
Doesn't really harm the fish, but causes the black spots.
There is a midwestern US species, another species spread on the US west coast, etc, etc.
I believe it is a similar parasite to the one that causes swimmers itch in humans.
I got it when snorkeling in snail infested lakes, as just a temporary irritant.
Lake Victoria is noted for a similar deadly human parasitic cycle that causes Schistosomiisis.