What can Ido about a returning heron? Just about all my precious fish are gone.

Alas, I now have the same problem. For me, all that worked was netting the pond all the time. I list some ideas at http://www.fishpondinfo.com/birds.htm
You can read about my attempts to save fish from herons in my pond newsletters from this and last year. The archive index is at http://www.fishpondinfo.com/news/index.htm