Which aquatic fern performs nitrogen fixation : -

Azolla is a small aquatic fern. The cyanobacterium Anabaena azollae lives in cavities of its leaves and fixes atmospheric nitrogen.
Azolla is grown in rice fields as a biofertilizer.
Nostoc is a cyanobacterium (not a fern), Salvia is a flowering plant, and Salvinia is an aquatic fern that does not fix nitrogen.
So the aquatic fern that performs nitrogen fixation is Azolla.