A few years ago, when I got in from work, my neighbour caught me, saying she had seen a heron standing in my pond.
I did ask why she hadn't thrown something at it,

Anyhow I had only lost 1 large goldfish, and maybe some frogs.
One or two of my fish now only shubunkins, is carrying a black gene, I have several black shubunkins, one or two are very large, with a slightly gold under belly when they come up for food they look like mini sharks!
The fish are very prolific breeders but a very few survive, the black ones mostly, I guess they can hide and don't get eaten like the lighter coloured ones do.
Anyway I digress, the next day at work I borrowed some litter netting, the stuff that surrounds landfill sites, 50 mm mesh and incredibly strong.
So much so it would support the weight of a small human without any problems.
I do occasionally see a heron sitting on the back fence looking longingly in the pond, but there is no way its getting in there