Black Ich, Black Spot, Tang Disease
Black Spot causes small black spots across the fish's body. The spots are very small, about half the size of a pinhead. They will cause the fish to flick and scratch and generally appear irritated as they would with the Ich parasite.
Black Spot is caused by the larvae of parasitic digenetic flukes, Cercaria and Metacercaria. It's a common disease in earthy bottom ponds and lakes, this disease is caused by a parasite (larval trematode) that burrows into the skin of a fish causing the formation of a cyst approximately one millimetre in diameter. This parasite has a complex life cycle that requires fish eating birds or mammals, snails, and fish at different stages in order to survive.
In general, even heavy infestations of these parasites do relatively little damage to the fish. There is some evidence that heavily infested juvenile fish may experience excessive blood loss, physiological stress, and even death. Also, fish with heavy infestations on the eyes may be blinded.
The term "Black Spot" refers to the formation of small cysts in the muscle and skin around which the parasite lives. The cyst accumulates black pigment cells. These cause little harm to the fish. Young fish have growth problems if heavily infected. The first intermediate host is a mollusc. The fish is the second intermediate host and the final host is a kingfisher or heron.
Black spot is generally easy to cure. There are a number of commercially available treatments and preventatives.
Always quarantine new fish.