It's not just heron that you've got to be mindful of either. If you live close to a river/brook, otter and mink will absolutely devastate a koi pond. No amount of netting is stopping either of those. I've had jackdaw take small koi and medaka out of my ponds as well.
Herons are a bird of prey, they prey on fish, and other things like frogs newt's, mise and even small birds in nest's. In my opinion, a shallow roof over the pond is the only way to deter em, also with a three foot picket fence around a low to ground pond, herons are cumbersome birds , so a gap of around two feet, between the roof and the top of the fence, should deter em, even a gazebo over it, and a fence around it, herons will land were ever they see water, so I'd hide the pond from them.
Herons are very patient birds and will stand for a long time very still, Koi are stupid, and although on the heron turning up they will go down, and maybe hide in your cave, but after a while they will come out of the cave and return to the surface, where the very stationary, still heron will be waiting, heron are very clever, Koi are stupid, heron are wadders, but they do sometimes swim, they can eat quite large fish also, but will stab the largest fish, just to get some meat, many people who net their ponds make the mistake of having the net too close to the water surface, it needs to be at least 2 feet above the water, without being able to sag under the weight of the heron, as they will walk over it and attempt to press it down to get closer to the water surface, I live 500 yards from a reservoir, lots of herons, but cormarants are worse, so for me a full enclosure was the only way, though my biggest fear is Otters which there are some living at the other end of the village, very best wishes, Tony, Farmoor Koi😀
ive just made a new enclosure for my pond ive not got a fish cave could b something to consider i fell out of love with my net over my pond even after seeing your tragic video
