Your tank is looking great, your bonsai is looking so nice, what type of moss did you go with?
These readings don't like too strange to me as has similar experince, I tested mine the other week and had 210 tap and 124 in tank, I have rocks in mine as well so not overly sure why it dropped so much?
Saw a tag about hairgrass, I've had it for awhile and growth seems slow I've recently added some very budget co2 will report if it improves, I would personally over plant it to start with if you want a lush looking carpet.
Also in terms of planting I would say it probably does need a nutrient rich substrate, soil is the obvious choice, is there anything under your sand that could give it that? Maybe liquid ferts would suffice but don't think you could get the good results as if it was in a more nutrient dense substrate.
Maybe someone a bit more knowledgeable can comment, I know
@Wills has got it in his current tank, our names are very similar I've just realised ha