Hi wallacevanpete, welcome to the forums.
What size is the pond and how deep is it?
I know little about ponds, but would suggest that whatever you do, do not empty it and scrub it out, there will be some good bacteria living in the yuck stuff in the bottom.
As Carlos & Siran have suggested, some surface agitation may be necessary as the oxygen levels in the water decreases as the temperature increases, so breaking the surface will go some way to increasing the oxygen levels, this could be why the fish are gasping at the surface.
Some members more knowledgeable in ponds may be of more help.