One possible experiment could be having the small rafts just have pots with soil and fiber and allow the aquatic plants to live for a season.
Try to find the source of the pollution rather than removing the duckweed. Rationale: Duckweed thrives on unhealthy ponds.
Try to remove the aerator from the big pond and place it in the small pond. Rationale: As said above, a possible source of pollution is the lack of dissolved oxygen.
03/01/18: Found out that the aerator is broken, nothing can be done.
Use nibblers like in 2016 and 2017.