You should expect to have periodic fish kills in a duck pond (shallow pond or deep wetland). I have three of these. They were intended to be fish free, but flooding stocked 2 of them and fathead minnows must have come in on waterlilies in the third. Enjoy them as wetlands or re-contour the pond bottom (deepen). Even well designed fish ponds can have fish kills due to oxygen depletion under ice or from decaying plant material.
