What’s wrong with miltoniopsis leaves

[Irisha99]

Hi.
Something is happening with my miltoniopsis. The leaf tips are turning brown and then it spreads it makes the entire leaf yellow. Is it the fertilizer ?
I keep it Away from windows. Day temperatures are in the 80-90F. I turn on the AC for the night and keep the room at 72F. Humidity is around 50-60%. I water it as needed, about every 2-3 days. Roots look nice and green. It’s growing in ceramis with sphagnum moss on top. I was fertilizing it every weakly weekly (I used 20-10-20). But since I noticed those brown tips I stopped fertilizing. So now just watering it with sink water.
Any thoughts what’s causing the problem? Thanks