Question
I brought the ten KOi in for the winter, they reside with 10 small goldfish in the 1000 gal pond. The pond is only 2 feet deep and we live in SE PA. So I thought I would bring them in for the winter to a 100 gallon aquarium. A month ago I visually saw the small goldfish start to eat the scales from my prime 12 inch kio. All the other fish including the koi started the same practice unit they had eaten to the flesh of # 1. I checked the water and all seamed ok and I clean the filter regularly.
I finally removed # 1 from the tank. The other fish had eaten into his flesh and continued to do so.

Now a month later # 2 prime is having the same thing happen. Even the other koi are picking at his sides to the point that the flesh is exposed.

I do have 3 small catfish in the tank as well but they have not been harmed nor attack the koi.

I feed them once a day. Am I underfeeding them. do they possibly have a parasite? what in the world is going on

Answer
How big are the goldfish, koi, and catfish?  You have 10 koi, 10 goldfish, and 3 catfish in a 100 gallon tank?  Even if they are all young, that sounds way overcrowded.  In crowded conditions, it's much more common for fish to harm each other.  Koi and goldfish normally don't pick on other fish but may certainly do so if a fish has an open wound or is otherwise sick or injured.  Even the most docile of fish see open injuries on other fish as an invitation to dine; even fish that they might consider their buddies.  They don't understand what they're doing.  The injury could have started from a physical injury, infection, or perhaps from the catfish.  Are they plecos, channel catfish, or some other species?  Plecos will suck on goldfish and koi in small quarters.  Channel catfish eat smaller fish outright.  I'm probably only a few hundred miles from where you live.  Here, my pond is only 26" deep, and I keep my goldfish and koi in it year round.  If you set up the proper system, you could probably keep them outside.  Plecos though would have to come in.  My page on winterizing is at http://www.fishpondinfo.com/winter.htm
For now, I suggest first checking your pH, ammonia, and nitrite levels to be sure they're ok.  I suggest changing 30% or so of the tank a few times each week unless the fish are all under 2 or 3 inches long.  Keep an eye out for injured fish or ones being picked on.  They will have to be moved to another aquarium or indoor pond to save them from being attacked/pestered.  Even plastic storage tubs work.  For fish under an inch, you can use net breeders to isolate them in a larger tank.  I suggest feeding the fish twice a day, as much as they can completely consume in about three minutes.  What temperature is their tank?  Lowering the temperature will reduce their appetites and waste load as well as how active they are.  Nothing you said seems to indicate parasites but since most fish carry some parasites, that certainly can't be ruled out, perhaps as a reason the fish that was killed was weak in the first place.  Parasitic medications are very stressful so I don't suggest using them unless you're confident that parasites are involved.  Good luck!

Robyn