If you dont want to loose what you have left in the pond, and there is a heron about. I suggest you get a net over it asap, or a wire around the edge of the pond as herons dont like stepping over things like that.Once they have paid you a visit they have a tendancy to keep comeing back.
As Peter asked How big is your pond? and can you see to the bottom of the pond in case they are hideing?
Best of luck, let us know how you get on.

__________________
Ex divers do it deeper