The same thing happened to me several weeks ago. A heron got several
of my 10-year-old goldfish. I understand that this is a natural thing
for a heron to do, but nevertheless, I hated losing my fish! The
remaining fish were spooky for a few weeks after their traumatic
encounter, but now they are back to behaving normally. I put some
chicken wire over the pond for about 3 weeks after the heron visit.
Unsightly, but it did the trick. After the water lilies covered most
of the surface of the pond, I took the wire off. Haven't seen any
sign of the heron revisiting, but I will probably have to put the wire
back on this fall after the vegetation dies back. They say herons
will return and continue eating fish until they are all gone. There
are a lot of ideas out there about preventing heron from getting fish.
The only thing I've tried is a physical barrier, but unfortunately,
it rates high on the ugly-scale.
Gary