help with yellowing phragmipedium leaves

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MrHappyRotter, the photos definitely show leaves turning quite yellow at the highest parts of their arcs. This is especially noticeable in the photo next to the sink. The yellow patches are mottled with green.

I don't know much about Phrags, but in other kinds of plants I would definitely suspect inadequate nitrogen, iron or magnesium - or a too-high pH, making the mineral nutrients unavailable. In other, nutrient-deficient plants, yellowing often happens on the parts of the plant most exposed to light. The plant struggles to replace chlorophyll damaged in the normal course of exposure to light if it can't get the necessary minerals and nitrogen.

blipe2, yes, Phrags are sensitive to too many dissolved salts in the water, and burn easily. It can take a while for plants to recover from this kind of yellowing since they need to absorb the proper nutrients. Phrags don't tolerate much in the way of dissolved solids in the water.

I would also focus on testing the pH of your water with fertilizer added and making sure it's appropriate for Phrags. You can get pH test kits at aquarium shops. The liquid kits are much more accurate than the dip strips. With the proper pH minerals are absorbed much more readily.

I don't know off hand the best pH for Phrags, but if you use the Search function in the maroon menu above you might be able to find out.