Aquaponics & Nitrogen Fixing with Legumes

1) Do nitrogen fixing bacteria form on legume roots grown without soil?
2) After a legume in an aquaponic system becomes dormant/dead, will it provide nitrogen to other plants in the system?

To the best of my knowledge, nitrogen root nodules can form on legumes in a soiless environment. The bacteria required to make these nodules may find their way into a system on their own, but they'll likely need to be intentionally inoculated. I'm not completely certain, but I believe that after the legume dies it's nitrogen would be released into the system. Aquaponics systems usually have all of the nitrogen they need from the fish waste, but planting legumes shouldn't really cause any problems if you pull them out afterwards.

I'm wondering if legumes can be used to lower the number of fish needed?

They very well may be able to, although I haven't given it much thought. Usually if I'm low on nitrogen I just feed my fish more