In California ..near the coast ..a net over the pond is the only solution to heron fishing. A friend lost almost all her koi until she covered her pond..I had a juvenile red tailed hawk perch in the dogwood a few times this summer. It is the only hawk I have seen here.
Personally..I’d rather not have a cover on a pond..I’d go the goldfish route!
It’s nature for the hawks to go for them and with all the miles of woods around us we will never have a shortage of predatory birds and animals. Oddly the raccoons have never bothered with the pond.