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I had a heron (first one I've ever seen in 5 years here) show up a week
ago and drastically reduce my fish population. Have a plastic net over
it now. He came back next morning, but was apparently unhappy with the
net and left right away.

Half Round Spanish roof tiles also make great caves for the fish to
hide under
(not the "S" style tiles) especially if they are out of racoons
reach...