Xenia also require very low nitrate levels, so in a way they act as 'test pigeons' in terms of water quality.
I'll offer a differing opinion, here ... When we had our 75g tank - cruddy skimmer, CC
substrate, always moderate nitrate levels, our Xenia grew like there was no tomorrow. We weeded it out weekly
. Moved everything to the 180g with a good skimmer, 'fuge, DSB
, etc., zero nitrate levels. Xenia was quite healthy, but didn't exhibit much growth at all
I've heard Xenia referred to as a "nutrient sink" - similar to Caulerpa. As a matter of fact, there are ppl who use Xenia for nutrient uptake in refugiums.