Kill the duckweed with Propeller herbicide or Tribune or Reward herbicide. If you don't you will be battling it for years and years. Even now it will be a small battle because it already dropped seeds 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).