We have just acquired a ranch with 2 fairly large ponds that are covered with duckweed. There also are quite a few large (6-10") turtles living in it. I think there may be some fish in it also. Is there something we could use to kill the duckweed without harming the turtles or fish? We also see herons sitting on the log. We are in Zone 5 or 6 in Oregon. Thanks in advance.