About a month ago, I purchased this Calathea Medallion plant at The Sill store in Chinatown. However, it doesn't seem to be very happy. The edges of the leaves are brown and brittle, and some leaves are yellowing. I know sometimes yellowing can be a sign of overwatering, so I haven't watered it in about a week. Right now the plant is on a desk, so it doesn't get direct sunlight but also isn't in darkness. I asked one of my green-thumbed friends for advice, and she thought initially it might be spider mites or type of water, but I checked the leaves and they seem fine and also switched to only watering with bottled water (but again, I haven't watered it in over a week). Anyone have any similar experiences or have any idea what to do?