From The Aquarium Wiki
About Carp Pox
It is thought that fish pox is mainly caused by a species specific virus like the Cyprinid herpes virus 1, or CyHV-1.
The disease begins as small, opal white spots with a waxy appearance and sometimes with streaks of black pigment. These spots stand above the skin about 1–2 mm and gradually increase in size. The spots merge so that large areas of skin are affected. This virus tends to show itself during the winter and early spring, especially in young fish kept outdoors. Despite appearances, carp pox is thought to be harmless, though it can be passed to other fishes. The lumps usually disappear as the weather gets warmer.
This is a virus and so there is no known treatment for this. If the affected fish are kept in clean, healthy surroundings the signs should disappear after 2–3 months. They may appear again but this is unlikely with healthy fish.