Yeah, didn't know it was fish. I have a 6 foot diameter koi pond, oh God I was pissed when a 50.00 fish disappeared. Don't know if it was from the ground or the air, but I put a canopy of netting over the water, and added lights to thwart night scavengers. It worked--

You can go to jail for killing them, call your animal control if all else fails, usually it's just 1 or 2 culprits. A barking dog is a good deterrent as well.

Good luck!