Probably the most serious problem for many ornamental garden pond fish keepers is the loss of fish to the heron. Herons are wading birds and mostly associated with wetlands. They prey on fish, frogs and invertebrates and obtain their food by wading through wetlands and visiting garden ponds.
Herons are a common site at the side of water where they stand on one leg with their heads hunched into the top of their wings. They are frequently seen along the side of the canals too (in fact where there are fisherman there are likely to be herons too). They pounce onto the unsuspecting victim in shallow water with their dagger like beaks. Once they have found a garden pond with a rich food source they are very persistent in hunting the fish. Control is not easy. The RSPB has a few suggestion on ways to deter herons from your pond. One rather radical suggestion, is to convert a fish pond into a wildlife pond with native plants and animals in areas where herons are causing persistent problems. Another suggestion, and one which many keen ornamental pond lovers will mention, is to add plastic mesh netting over the pond, or providing the fish with cover to hide underneath. Human presence is also believed to be a good deterrent so perhaps you could make an exact replica (a scareheron!) of yourself and secure it at the edge of your pond. If anyone has any other advice on how to deter herons, please share this with us.
As well as stealing fish, herons can accidentally pierce flexible pond liners with their dagger-like beaks. If you are planning to construct a pond to stock fish, make sure the flexible pond liner is protected from such potential accidents by laying a protective pond liner over the liner as well to the underside.https://www.pondlinersonline.co.uk/