The lower leaves on the phalaenopsis turn yellow with stains quickly

[Shadeflower]

I don't think it is dying.
I don't think there is anything wrong with the pot.
5 leaves dropping is not normal..

Personally I would still be fertilizing especially if keeping it at 20 degrees, that is summer temps... Phals don't rest. At least warm growing ones don't so if you keep it warm with additional light my motto is to keep on fertilzing it.

Could be as simple as that. Literally.

I'm not a fan of tap water myself and I believe long term it contributes to leaf problems but maybe I am exaggerating, it does depend a lot on the water quality. Some people have soft water and no problems using it. Rain water is just safer imo