My Phalaenopsis has been sitting on the windowsill for about half a year and looked really good until two weeks ago. Now I'm getting more and more brown spots on the leaves and I'm not sure if I'm watering too much or if it's some kind of fungus. The humidity in my living room is pretty low, and I use tap water.
With Phalaenopsis, brown spots are usually a combination of overwatering and poor ventilation, especially if the humidity is low - it's more of a classic root rot issue than fungus. I'd cut back on watering for now and only water when the substrate is really dry, and take a look at the roots - if they're dark brown and mushy, then it was definitely too much water. Tap water can also play a role (chlorine, salts), so try using rainwater or distilled water instead. If the new leaves grow in healthy and the old ones stop getting affected, you've got it under control.
Brown spots on Phalaenopsis are usually from overwatering, especially if the humidity is low - that's when moisture gets trapped in the substrate even though it feels dry. With tap water, calcium deposits can also be a problem, but that usually looks more like light spots. Take a close look at the affected areas: are they soft and mushy or more hard? If they're soft, then cut back on watering significantly and let the orchid dry out almost completely between waterings - I only water mine every two to three weeks, even with the best light. If the spots are hard and not spreading, it's probably just calcium deposits and not a big deal, but with damp substrate in winter, real fungal infection can show up quickly, so pay attention to ventilation.
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