A horde of goldfish or shubunkin will do a good job of nibbling duckweed and whittling it down to size, before it gets going in Summer.

When they have done their bit and their population explodes, expect nature to do the dastardly deed and send a plague of predators to thin them out...

Some sort of mesh should stop those fish from being flushed downriver when storms overflow.

Duckweed, and many other opportunist aquatic can literally hop from pond to pond, on the back of a frog, or a bird

Regards, andy
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