Birdsnest Tip Turning Brown

[Nanobros]

My pink birdsnests tips are turning slightly brown and I don't know why. My phosphate is at 0 and my nitrates is at 2.0 ppm. I did a water change last week. What can I do to fix this so the situation doesn't get worse?

 

[RyanCSGO]

what are your other water parameters?
ALK:
CAL:
Salinity:

are you dosing anything to maintain these?