Thanks for your help, tropicbreesent. I will repot it with an orchid mix and try just keeping the soil slightly moist, rather than watering deeply and letting it dry completely between waterings. Hopefully this does the trick! (And do you think it is worth it to keep a pebble tray beneath for added humidity in the air?) If anyone else has thoughts, I'd welcome them too! (Including about where I could buy another of these plants!) Thanks again!

Mine are all outside but when they get a bit too dry and stressed some of the older leaves will develop that sort of blotchiness. I think the plant decides it has too many leaves to support and begins the process of shedding some of the older ones. I never fertilise mine, as semi-epiphytes they don't need much. Of course a bit wouldn't do any harm, especially if they're in pots with restricted root space. In a garden falling leaves and insects are enough.

The plant in the original post has been over-watered and perhaps over-fertilized or suffering from a high level of salts in the soil from tapwater or fertilizer solutions - same with the plant in the first image upthread. Some growers think because in situ plants are often found growing in riparian settings (streamside), they prefer or enjoy overly water-retentive media. The plant can adapt to inundation by restructuring its root structure (to deal with either terrestrial of bog settings), but it's unable to deal well with transitioning back and forth between conventional container culture and some form of hydroculture. As long as a plant hasn't contracted one of the damping off fungal (root) infections, recovery depends primarily on the cultural conditions it's asked to deal with. If those conditions are near or beyond the limits it's genetically programmed to deal with, the plant will do poorly. If cultural conditions are all 'in the sweet spot' it should reward the grower with good growth and the vibrancy that comes with a high state of vitality. A fast draining medium you can keep evenly moist (not soggy or saturated), room for roots to grow, an appropriate nutritional supplementation program, plenty of warmth and bright light are all conditions for which your plant will show its approval. Al

Do you have photos of it? Use a fungicide if it looks like fungus and make sure that the leaves arent staying wet. If possible, remove infected leaves.