Rather than using, nitrate, NO3- as its source of nutrients it uses, ammonia, NH4+. Your hydroponic nutrients probably had ammonia, NH4+, in Ammonium Nitrate form, NH4NO3 (note the numbers shoud be subscripts). This means that the ammonia is locked up with the nitrate instead of being with on its own with another element, this would make it attianable. If you want the duckweed to grow you could put some urea or pure ammonia in it, small quantities.
David Schwinghamer said:
Please explain the last part of your reply