Aaron, when using Sonar at a quart per acre you will kill every plant in the water including the duck weed. This can cause the water to stain. When going after duck weed I would wait until until the pond is completely covered. You can sometimes only kill whats there and when the product is completely gone or the ppb levels are too low to kill duckweed you will see regrowth. Sonar or Avast can only kill what is there and is not a preventative.