Plants in an aquarium do not suck up lots of nitrates because nothing in their environnent is maximised. To maximise the growth of a plant, four things need to be maximised : the lighting, the O2 at the roots level, the CO2 in the air, the nutrients level. If one of these is deficient, the growth will also be.
For exemple, a non aquatic plant receiving direct sun light will be able to consume suprisingly lots of nutrients (nitrates ...) comparatively to a plant placed in the bottom of an aqarium lighted with t8.
I would be very curious to see the results if someone could place a big plant with very high growth rate, let say a tomato plant, with the roots immerged in a sump, near the intake to maximise O2 at roots level, lighted with high pressure sodium or metal halide and CO2 injection.
I know it would be costly but it would produce almost 0ppm nitrates and good tomatoes !