Fish disease

Question
WE have an outdoor pond approx 11,000 gals, we have never had any problems until last year when some of our fish, mixed, shubunkins, goldfish, comets etc got what looked like large ulcers and started to die, we took some to the local supplier who tested them and the water and said we had fluke.  We treated it and waited, did not really improve, so asked again and again they said water was OK, but we still had fluke.  So we treated again, seemed to improve a little but we lost more fish.  It is now February and has been a very cold winter, we have kept filter running all time, but looks like we have still got something, as some of them  have big eaten looking sores again.  We thought if it was fluke the cold would have killed it anyway.  We don't know what to try next, any suggestions.
Thanks

Answer
don't listen to those people any more.  Fluke is one of the most easily identified fish diseases.  The reason is that they're bright green little "water-tick" looking critters.  The problem you have is a little harder to identify and my guess is that your local fish store didn't test enough parameters.

Your problem is quite obviously a bacterial infection.  This isn't the end of it though.  Those bacteria will often be present in the water, but unable to affect healthy fish.  A fish's immune system comes in the form of a slimy mucus that they produce to coat their scales.  A number of different problems with the water can eat away at the fish's slime coat, leaving it vulnerable to infection.

The water conditions that can lead this are:

high/low pH and pH swings
extremely low KH (this allows large pH swings to occur daily)
High GH (this also directly affects the fish's ability to regenerate its slime coat)
High concentration of ammonia or nitrates
I'm sure there are others as well...

Follow these steps:

1)  Try to correct the chemistry problem, if you can't don't get hung up on it.  Keep with the other steps, but don't ever slack on the chemistry.

2)  Get a product called Fu-Ba-Fix or some other version of a anti-bacterial liquid for koi ponds.

3)  Once you can see the fish looking slimy again, just keep an eye out for other signs of stress (clamping fins, streaking, red fins) to make sure they haven't contracted parasites as well.

4)  Be careful when selecting new fish.  Many fish carry bacteria on them from the store.  If they come into contact with another fish that's not used to that bacteria, everyone gets sick.