Kitoh, etal. 1993. The growth and nitrogen fixation of Azolla filiculoides in polluted water. All of these articles have lots of citations. There seem to be a number of qualifications however. I was wondering if anubias might need nitrates also.
Aquatic plants do very well on nitrates.
They all grew very rapidly and used up all the nitrate supplied. This is not to say that they can't also use ammonia. I believe that if you have measurable nitrate, even as low as 1PPM, that should be plenty for your plants. If the plants aren't doing well, look to other needs, nutrient, or otherwise. With UGF, iron deficiency is likely to occur.