Since they are taking plankton size bites, I don't think they would appreciate all the inedible fibres that would result in passing raw duckweed in a blender...
Boil it until the soft tissues of the duckweed detaches and remove all the rest, stems and leaf vein... You can also add some finely crushed fish flakes to the mix when hot and let it cool. To try to encourage them to try it.
Boiling disinfects and permits to have a much finer product... You should be able to conserve it refrigerated for 7 to 10 days. Dissolve in a little bit of tank water to temperate before serving.
Keep us posted, I'm really curious