Hi all,
You could try Azolla, but it usually does best in bright conditions, it will also be green rather than red in low light.
Lemna gibba (Fat Duckweed) and Salvinia natans will grow in shaded conditions, and I've also found Limnobium (Amazon Frog-bit) will persist, although it may eventually dwindle as runner formation seems to be suppressed. I've got plenty of these spare if you would like any of them.
cheers Darrel