With UGF, iron deficiency is likely to occur. I grow my plants in soil or a soil-gravel mix, with no filtration of any kind. I am sure that Anubias can use nitrates also. Anubius is one type of aquatic plant whose roots do not appear to be adapted for living in anaerobic mud. I have noticed that Anubias does almost as well floating as rooted, and that Anubias roots tend to attach to stones and gravel the same way that the modified roots of ivy cling to stone.
Aquatic plants do very well on nitrates.
I think I can add some information on the ammonium vs. nitrate debate. About a year ago there was an article by Diana Walstad that said something to the effect that aquatic plants preferred ammonium over nitrate. This floating plant was grown in a mix of equal concentrations of ammonium, nitrite and nitrate. Tests showed that the ammonium was absorbed first, and nitrate last. If nitrates read zero, you probably should be adding some nitrogen.