If a Cyperus gets bad (yellow) leaves this can have several causes. It's important the Cyperus is always under water with wet feet. It's a real water drinker.

If you place the Cyperus in front of the window, this can also be a disadvantage because of the cold in winter or heat in summer. The plant receives a blow from the temperature fluctuations. Put the plant in a different place and rotate it regularly so it gets light from all sides. In the worst case, the Cyperus loses some leaves that grow back around summer.