I believe the black patches on bass are caused by a virus. The black spots mentioned earlier are different- they are small, mole-like markings caused by a parasitic trematode. Both are usually harmless to the fish. Black spot, like many parasites, requires more than one host species to complete its life cycle- in this case an infected fish must be eaten by a kingfisher, then new fish may be infected by trematodes in the bird's feces. So it won't spread in your tank.
