one possibility, though not a very likely one, is diplopstomiasis or 'black spot disease'. many kinds of fishes may be found to have tiny black spots the size of a pinhead up to 1 mm in diameter on various parts of their body, such as in their skin on the head and body, inside their mouth, on their gill arches, and on their fins. The black spots are caused by the adult cercaria life stage of parasitic flukes, which are digenetic trematode worms, occurring when the mature female penetrates the skin of the host fish and lays or secretes thin flexible egg sacs called cysts.
the life cycle of this parasite is generally far too complicated to occur in the common aquarium situation. two intermediate hosts are required -- fish eating birds, the definitive host, usually carry the mature flukes in their intestines and drop the eggs into ponds as part of their feces. these eggs hatch into free swimming larvae which infest snails -- the intermediate host, and develop into additional larval forms. these larve then infect fish.
the most likely scenario is that these markings are part of your fish's normal coloration.
in any event, if your fish do turn out to have "black spot", this disease is non-invasive and usually clears up on its own since without the required intermediate hosts, the life cycle cannot be completed.
should you decide to treat .. the medication of choice is Kordon's Trifon, containing trichlorfon.
