I stick to using Sonar A.S.

For duckweed, if labor is free, (you doing the spraying) I'd use Clipper. The concentration of Fluridone needed to hit duckweed will also wipe out any other vascular plants in the pond.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).