Noid Phal losing mid growth leaves

[greenpassion]

I have a small Noid Phal that is losing leaves that are not the oldest leaves. I didn't think anything of it at first, but now there's another one turning yellow. It is not the bottom most leaf which would be the oldest, it is the ones just up from there. I grow these in Orkiata bark, and clear plastic pots, I water them with well water and once a month I do the kelp Max and and Inocucor. I have well over 20 phalaenopsis orchids, and everyone is super healthy. This is strange. Any ideas?