A description of herons and garden fish pond protection, from RSPB
https://www.rspb.org.uk/get-involved/co ... sandponds/
A pond in your garden is attractive and provides an extra habitat for wildlife. A pond with gently sloping sides will be used by small birds for drinking and bathing, and may attract toads, frogs and newts, or even herons.
If you keep ornamental fish in the pond, you may prefer not to receive visits from a heron.
Herons eat mostly fish but also take amphibians and small mammals, with small quantities of reptiles, insects, crustaceans, molluscs, worms and birds. Herons fish mostly at dawn and dusk so they are rarely noticed.
Young herons teach themselves to fish, and when they leave their nest in June and July, small garden ponds are attractive to them because they often provide easy fishing. Quite understandably, herons will respond to a garden pond in a comparable way to a blue tit being attracted to a nut feeder.
There are many ways that a garden pond can be made less attractive to herons, but remember that many of the ways of ‘heron proofing’ a pond will deny its use to other wildlife also. If you have/are likely to have a persistent heron problem, it might be preferable to forget about the fish and turn the pond into a thriving wildlife pond, where the heron would be a welcome component.