I had a pond years ago, bought new fish after cleaning it out one summer...came out a couple of hours later, looking in the water and can't see the big koi I just bought? wtf.....turned round and its sitting on the grass next to a bush
grabbed it, and chucked it back in the water and it darted off...phew!
couple of years later fish started dissappearing, so stuck a net over the top to stop birds/fox's getting at them, then eventually got rid of the pond sadly.