Xenia remove DOMs, so it will it will lower NO3 and PO4 indirectly. One caveat is that the Xenia must be removed periodically. I remove 7-10 stalks a week in my heavily stocked 300 gallon system. I've been almost completely skimmerless for over 4 years. It has worked wonderfully, and as long as you keep it from getting into your DT, and you've figured out the balance between nutrient import/export. It will work as a very effective filtration source. In the wild Xenia is one of mother natures many nutrient removal systems. They can be found near hotel waste water outputs as well as other nutrient rich water.
Here is a video of my Xenia refugium
https://www.youtube.com/watch?v=BKfhOu2GBaA