help with yellowing phragmipedium leaves

[blipe2]

I am hoping for some help with my phrags. I moved to upstate NY in the summer and this is my first winter growing them under lights. They have all started to get yellowed leaves that are yellowing from the middle outward. I wasn't too worried because they are growing and one is even in bud, but now the new growth leaves are also starting to get yellowed, so now I am worried. They are under a bank of 8 fluorescent lights. I use only RO water. I fertilize about once per month because I was starting to get some blacked tipped leaves when I was fertilizing more often. I run a humidifier during the day. The temp is high 60s during the day and low 60s at night. I thought it might be too cold so I moved them up higher under the lights, only about 8 inches below the middle tube. I am hoping for some thoughts?
thanks