Cattleya (Sophronitis) coccinea with yellowing leaves at the bottom

[alfrsa]

Hello everyone. I have two Sophronitis coccinea growing on kool-logs and they seemed to be doing fine, but I noticed one of the plants' leaves started yellowing at the bottom, near the pseudobulb.

Both plants have it, but the one on the picture has it worst. It's putting out two new growths (one visible to the left), but had little to no healthy roots when I got it about a month ago.

I grow them under lights, in temperatures of no more than around 20°C (68°F). I have a fan on a timer about half the time, and I have kept the kool-logs filled with distilled water, letting them dry for about a day before refilling. Humidity ranges from 60% (fan on) to 85% (fan off at night). I fertilize with Akerne rain mix at half-strength.

I've looked around online and found a lot of photos of S. coccinea flowering with yellowed leaves at the bottom, but I found as many photos with plain green leaves.

What do you think? Is this a nutrient deficiency? Too much water? Too little light? The yellowing part is firm, but given this species' reputation for needing very specific growing conditions I'm a bit alarmed!

Thank you in advance for your help.