Not necessarily a heron, but almost certainly predation, given the behavior of your remaining fish. Cape Town is on the seacoast, so ospreys and other fish-eaters have found you. It might be worthwhile to cover the pond with a hoop-house arrangement and shade cloth to hide the presence of water from the air. The herons already know you're there, and the only things that work for them are either water too deep for them to wade in or effective anti-aircraft fire.

Bob Passovoy