Sorry for your loss. I had the same problem for several years. I know this will sound silly & much to simple, but it has worked for two years for me. It has worked for everyone else that I have told also. I do keep a net over my pond just in case something other than Heron try to get my fish. But the Heron will not even come close to my pond any more.
Go to Wal-mart or Target and buy Pin Wheels in the toy dept. The shiny ones made out of foil looking material. They come in different colors. They spin with the slightest breeze and reflect light all over the area around the pond. Two things Heron do not like is fast movements and shiny objects. I placed mine about every four feet apart around my pond. The Heron will land close to the pond but will fly off when the pin wheels start moving. My little GD had two, and one day wanted to put them by the pond. That is how I saw with my own eyes that the Heron were afraid of them. So I went to Wal-mart and bought more.
They are about a dollar each. I told my family why they are there. I didn't want folks to think it was my decorating ideas.
They are easy to pull up when you have company at your pond.
For a dollar each, give it a try and let me know. It has worked for many here that I have told.