Hi Dale Im starting to get the jist of your problem. More info at the begining would have been helpful. I take it you had these koi over the winter. Now it is spring and they are comming up with problems. Without seeing the pond I will have to just give an opinion. I have seen this before. You have a Fungal desease in the pond. It will not be cleared up when the water is cold it has to be over 70%. The cold water is going to make the fish susceptible to fungal attacks. You have to wait till the water warms up if it is not in order to treat the pond and fish. Your pond cemistry is ok and that is not the problem. I think by the size you gave me you have 2500 gallens. Potassium Permanganate has a 50% chance to fix the problem if the water was warm enough. Now this is what I would do I do not tell people what they should do. When was the last time you changed the water or cleaned the pond. If its more then a year the bottom of the pond has a field of muck which is infected with all kinds of bacteria . That muck has to come out and the pond The water drained and dumped and the pond desazitized. Its the only inexpensive way to get rid of the fungas. Put the koi in a hospital tank and warm the water and treat them. I see no other fix or cure. I empty my pond and clean it and put in fresh water every spring. One other thing do you keep the fish out all winter and the pond freezes over do you kep a hoe in the ice for them to breath? Now it sounds like a lot of work It hakes me a couple of hours todrain my pond an hour to clean it then start filling it. I let it condition for a couple of weeks before I put the fish in it . I have a 12 thousand gal pond 14 koi that are 10 to 15 lbs and are 24 to 30 inshes long and I bring them in every winter. I dont see that you have any choice There is no guaranty that you can fix the pond. I know itb is a lot of work but its do able. I repeat this is what I would do. let me know. I hope I understood your problem. I stll would clean out the pond . I have been doing it for over 30 years and never had a sick koi.