Hello, me and my family are building a Koi pond, and then we are going to move in Tilapia. These fish are going to be eaten. Is it okay for ducks to swim in a pond full of fish that we are going to eat? Thanks. P.S. How come ponds without water valves or being filled in any way don't evaporate? We have 5 ducks, 4 magpies and 1 pekin.