It may be a nasty Herron. I just had one pay my backyard pond nestled close to my house near my kitchen a visit, twice in the same day! It cleared out all my beautiful fish, including my 3 favorite Koi, Spot, Big Red, and Silverwing. I am totally devestated and I went out there to check this morning and nasty ugly bird was back! I chased it away. I have a single frightened goldfish left, hiding in one of the hideaways I had in the pond. So I threw a bunch of plastic tarps over the pond to dissuede the bird until my net arrives. I am hoping the herron steps on a floating tarp, causing the bird to immediately sink and the tarp wrap around it. Maybe it will be freaked out and never come back. As soon as I get my net I am stringing it up right away!