You may want to try a Heron decoy. If you put one out and move it every so often they will not visit the pond. Herons are very teritorial and will not visit a pond that is someone elses turf. It has worked on many ponds up here in NY. The decoy usually is about $60 but alot less expensive than restocking and trying to re-balance your pond.

As for the Great Egret, I do not believe they are as territorial as the Heron and am not sure how to keep them away other than an electronic eye tied to a noise making device such as a air cannon. I haven't tried such things because up until recently we have only had the Blue Herons up this way. I have seen one Egret up in the area now and am hoping I don't have to go that route myself.

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1/10 - 1/4 acre pond plus 16 ft deep/ Plus 40 ft by 20 ft by 6 ft deep koi and fathead minnow pond next to it. Upstate NY