Its a real problem, my fish jump quite high, and friends who net find the heron just pokes a hole with that sharp beak. :0(

I,ll watch these answers with interest

a minimum of a foot. put the netting over the egde into the surronding 'garden' as they wont get tangled. They cant stand on the edge to stab at the fish.

I,ve read that Seaburn but our koi pond had a low wall around.....about 4 bricks high.....and the pond itself is very deep, five feet or so, and we lost young koi (no bodies)
either the heron ( who we see)
or the ghost koi are cannibals which has been suggested
What do you think?

I use childrens big windmills as a deterrent which works, but only when its windy.

I once read to make the netting into a fence around the pond rather than spreading it over it which is supposed to deter them from landing. Probably needs to be higher than four bricksworth though.

Electric fences too have been suggested but a bit unsightly, they are wading birds....
They spit on tbe pond to attract the fish, then snap. :0(

Having thougt about both the bird and the garden owner,
wouldn't chicken wire spread over the pond be safer for the birds as they wouldnt get caught up in this and you can buy it in green to blend in.

Like steragram, I was advised to make a kind of tripwire about a foot high all around the pond, the theory being that heron wade into the pond to feed. It did work in my case.

Heron certainly land and wade. they suffle their toes in the mud to startle the fish then spear. Never seen or heard of a heron spitting. the dummy heron decoy works well.

The best method I have found (I accept there may be others), is to string black cotton at about a foot to eighteen inches at intervals on the approach to the pond. That prevents the heron from walking to the pond (they tend to land close and wade in). Aesthetically, too, the cotton blends into the background. Of course, if there's an island on which the heron can land, then cotton won't work!

Thank you all so much for the comments and suggestions. We've settled for netting stretched across the pond and held up by short stakes about 10" high.
We might go for the fence idea, using netting around the sides of the pond at an angle of say 60 degrees or so.

No need for an angle - the fence is normally vertical.