Mitch your pond is quite deep and very large, so it is behaving the way a natural lake would, warm water at the bottom, cold at the top.
Wayne, did you go take a look at the pond today? Did you find any other fish with the fungus? I read that attachment on the fungus, and it says that basicly the fish were breathing in ammonia and got stressed which lead to fungus. A very clean pond is more likely to have toxic levels of ammonia, and here is why, when you change the water, you raise the PH level, and our water here in town has a very very high PH level, and that is from the South Saskatchewan River. I know you got the North Saskatchewan river, and I would bet that if you test the water comming out of your tap, you will probally find that the PH is well over 8, and close to 9! A high PH is not good for pond, as you will get toxic levels of ammonia with the high PH.
Pond Peat , or natural leaves, will lower the PH so that ammonia will not become toxic. When you use liquid Pond Peat, the water will darken to a golden color, the leaves do the same thing, turn the water a nice golden color also. Natural ponds come with leaves, people write stories and books about "on Golden Pond", as natural ponds are natually golden. so why do we try to keep a "sterial hole in the ground", and call it a pond? Real ponds have large sedges, leaf litter, snails, leeches, and other aquatic life, and I try to keep my pond as natural as possible, and after many many years, I found it to be the best way to have a "naturally" clean "carefree" pond.
if you turn your heater off to stop the growth of the fungus, the pond will freeze over and the ammonia will build back up, and the fish will go back to breathing in the ammonia. Then the cycle will repeat itself when the ice defrosts., and next time the fish will be even weaker.... If you leave the heater running were the water flows well, the heater will evenly heat the whole pond, this is very important, as you don't want to have large temperture swings from the deep end to the shallow end, as that can make the fish sick too. I always stress the point of having the heater where the water flows well for that reason alone. Once the heater has been on for a couple days, the ice should thin out and melt, exposing a large open hole, for the sun to shine in and then the built up gases can escape freely.
Charles, Wayne takes very good care of his pond, and cleans it very well, and I don't clean my pond at all, so I should have the Fungus according to your theory, but I don't. As I have said before, you can "clean your pond to death", which I used to do, a very long time ago.... and that created the problems! Now I don't disturb the fish at all before winter, and I leave some leaves in the pond, and I have some sludge at the bottom pond, and that's OK, cause it's all natural and provides a home to the snails, leeches and good bacteria. (They all work together with the plants)
Wayne, I can mail you pieces of my pond... a box of yellow flag roots and other plant shoots from my pond in the spring if you like.
Cheer up, it's not the end of the world!
