Hi Pmac. One of the questions I would ask is whether you had sufficient fiiltration in your pond and do you make monthly water changes from the bottom of the pond to remove excess toxins.
Do you have rocks, stones or gravel at the bottom of this pond?
How many gallons is the pond?
How many fish were in the pond?
What type of filtration are you using?
What type of food do you feed and how often.

Yours Koily, Lorraine