Years ago we put in a 15x15x3ft deep triangular pool in a fence corner at school, had a recirculating waterfall and everything. But it had to be self sufficient so we only put two cheap Walmart fantail goldfish in there along with cattails, water lillies, some algae, topminnows etc. Didn’t take long for those goldfish to grow as big as softballs.
All was well for some years until one disappeared, so I sat and waited some distance away the next evening. Sure enough a great blue heron came in low over the school, folded up and descended upon the pond, a moment later it stepped out holding the other fantail, beat it against the ground once or twice, pointed its beak up and swallowed it. It looked like a softball again sliding down the heron’s neck.
Had to take that pond out, liability, it had to be 3’ deep or else it would have overheated in the summer.
