Plants can not utilize nitrogen gas, N2. That's too bad, because 79% of the atomosphere is N2. They must get soluble forms of nitrogen, such as ammonia (NH3), nitrite (NO2), nitrate, (NO3) or urea (CO(NH2)2. Legumes and other plants that fix nitrogen live in areas where soil nitrogen is low. Their symbiotic bacteria "fix" atmospheric nitrogen (N2) by converting it into soluble, available forms, such as NH3. The process takes a lot of energy that the plant partner must provide. Legumes can use as much as half the energy they capture in the process of photosynthesis to feed their symbiotic bacteria and keep conditions just right for them. The payback is that they get nitrogen fertilizer where other plants can't. They can grow in low nitrogen habitats where other plants only struggle.
As Dennis said, the only known N-fixing aquatic plant is Azolla, which partners with the cyanobacterium, Anabena . Azolla is known to provide valuable nitrogen fertilizer for rice fields. I think what Adam wants is a plant that stores excess nitrogen fertilizer so that it isn't wasted. That is different from nitrogen fixing. Actually all aquarium plants can store excess nitrogen. They can store in their tissues over twice the nitrogen they need to grow at their best rate. If the supply of nitrogen is cut off, they can more than double their size before their supply of stored nitrogen runs out.