Handsome young fella!

I've got to say that not adding fish is the best way to avoid heron conflict, having said that they prefer frogs anyway so it's no guarantee. Nets are the best solution, although the sprinkler systems with an infra-red sensor are great as long as you remember to turn them on after you've turned it off to avoid a soaking if you're working in the garden. Plastic herons work if you aim them right but unless you're throwing one it doesn't do much.

Goldfish never get too big for herons to swallow (they can take 12" carp) so unless you saw one discard a caught fish I'd suspect something else and there's only one predator common in gardens that doesn't need to eat what it catches