We bought this ScareCrow contraption and set it up to protect our ponds at night. It shoots out water at high-pressure whenever something moves within range of the pond. Surprisingly, we got it for much less at Green Thumb Nursery - the place that normally costs much more than any other place! Hopefully, our three new koi and the goldfish in the lower pond will be safer now.
(If you click on the pictures, you can see the fish in a bigger format.)