Hi Craig,
Could anything be polluting the water? Is the fresh water acceptable? what are the water parameters.Is your pond visited by birds(often they carry diseases).
If its bacterial you need expert advice and or veterinary advice. May need a swab .
Think that you urgently need professional advice. Paula Reynolds at Lincolnshire Fish Health tele 01205 723413 daily from 10 till 4 would be my go to.
Tony