First, are those black lines I see color on the fish or hanging off? If they are hanging off they are probably anchor worms or leeches, a little hard to tell with the pics. If they are anchor worms you will need to quarentine all the fish and take them all out of the pond for a few weeks, as they need a fish host to live. They will need daily water changes, and substrate and plants should be removed from the quarentine tanks. If this is not possible, you may want to look around the internet to see if there is a chemical cure that can be added to the pond.
Second, it looks from the physical damage like you probably have a water quality problem as well. Does your pond have a good filtration system? Have you recently checked your ammonia, nitrites, or nitrates? If you don't have a kit you can take a sample to your local pet store and they'll do it for free. This could also be caused from overstocking. If it is overcrowded they may pick at each other, and the wounds become infected and spread over time.

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