HI Juggy. I suspect that the water conditions are bad in the pond and that's why the fish are getting sick with sores and dying.
The first thing I would do would be to make a one third water change from the bottom of the pond and stop all feeding of the fish. It could be that your filter isn't sufficient for the size of the pond or the fish in it and/or you might have to clean it often which means that it's probably mechanical with no good bacteria to remove the toxins from the water.
I've got a great diagram of a downflow bio-filter along with step by step pictures if you are interested.

Your's Koily, Lorraine