Hi Odabel, and welcome to the forum!
It sounds like you've made quite an effort to keep your new guys happy! It's natural for orchids to occassionally drop their oldest leaf (the lowest on a Phal.) It's part of their growth cycle, and you shouldn't let it freak you out (that's always my first reaction!) But that may not be what's going on here...
Yellowing like that sounds like a moisture problem - particularly if the potting material feels wet. Yet having only watered it once in nine days isn't much. Tell me about the stones the plant is sitting on - is the water in the dish below the surface of the stones? That is, are the orchid pots touching the water at all?
If so, even though the water is below the roots, the potting material will wick the water upward through the pot. That could explain why the potting material feels moist. I'd say with the ambient humidity in the room, you won't need stones and water in the dish at all.
It's also worth mentioning that any sudden change in conditions can shock a plant. Unless you bought this plant from a humid greenhouse, it may be a big change from what it was used to. That tends to be more of a problem with blooming plants, where the sudden change can cause the buds or blossoms to 'blast'.
If the pot's not sitting in water, you may want to consider unpotting the plant and checking the roots. It's very common that you buy an otherwise healthy looking plant that's been overwatered before you get it, and the roots are rotting (which will cause the leaves to yellow and die.) If nothing else makes sense, I'd check the roots. They'll tell you definitively what's going on.
Finally, does your spa use any chemicals, like chlorine? I've always wondered if pool/spa chemicals effect the health of nearby orchids. I figure if we can smell the chemicals, the plants can too. Anybody on the forum have any thoughts on this?
Good luck and keep posting!