Posted 28 August 2009 - 05:30 PM
Yep, your fish have black spot (black grub) disease caused by a larval digenetic trematode typically belonging to the genus Diplostomum. The good news is they are mostly harmless even in very large quantities (your fish seem to have an extremely minor infection).
The definitive host is a typically a fish eating bird like a heron, kingfisher, etc.... which passes the parasitic eggs in its feces and then goes to an intermediate host of a freshwater snail then into your fish where they migrate to the scales. You can easily treat it with copper sulfate but frequent water changes and time should allow the larvae to die off and never be seen again unless you have fish eating birds visiting your home aquaria. If you have live plants in your aquarium do NOT use copper sulfate.
Best,
Blake