hygrophila emersed hygrophila has grown out into the air. Will that help eliminate nitrates
In fact, it CAN help with elimating some nitrates. It might even grow a little better since it would have more access to CO2 from the air. You'd be surprised at how much it is able to consume. This, of course, increases the oxygen in the area, as well. Most of it needs to stay submerged, however.
https://www.gardeningknowhow.com/ornamental/water-plants/hygrophila/hygrophila-plant-care.htm