Ok I'm putting in new substrate with root tabs and liquid fertilizerIt's a really difficult plant to keep without premium substrate and good lighting. It needs a very rich soil to do well. I'm excellent with plants and it's one of the few species I've struggled with.(really the only one)
How much should I cut it backThe brown dying part is it's emmersed growth; this plant was grown only partially submerged in water. The stalks are actually capped with flower buds.
The emmersed portion will die back, and it is best to actually cut the plant back to encourage it to put out new growth.
Dwarf hairgrass is not for beginners. It likes a fine, nutrient-rich substrate, medium to high lighting intensity, and prefers an environment rich in CO2. If you cannot provide those conditions growth will be slow, stunted, sparse, and the plant may just die. If you have sand I suggest root tabs. If you have gravel, then, to be perfectly honest, don't expect amazing results even with root tabs.
How much should I cut it back