Light green/dark green splothes?

[greenpassion]

Wow. Here I go with yet another issue. I have 13 phals that are growing under my led track light garden (from Orchid Web)
I have had them here for ages, with no problem. A few months ago, I had noticed that one of them had 2 leaves that were developing areas of a lighter green. Around that time I changed fertilizers from Dyna Gro to MSU for well water. And as soon as I made the change, the new growth was all uniform in color. It's put out 3 solid green leaves since.( The ones that were splotchy were the ones close to the media.) I have read here that when plants are lacking in nutrients, often they will develop this color issue. That said, yesterday, as I was watering, I noticed that another one is doing the same thing, but not older lower leaves. These are newer leaves. I will post a pic, but I don't really think it'll show the discoloration, as it's not glaring, but more subtle.

---------- Post added at 02:40 PM ---------- Previous post was at 02:20 PM ----------

I meant to add that I have them on a heat mat, which I had not turned down when it got hotter here. (Vermont) So could it be they are too warm?