Two thoughts:
Is the gravel on the surface of the pot glued together? If so, break it up and pick it off. That will hold moisture in. My experience with Home Depot is that they don't care for plants at all so I suspect over-watering may not be it.
Are the plant affected plants the ones closest to the window? If so, it could be sunburn as these plants are grown under lights. Sunshine is a foreign subject so plants need to be introduced slowly to sunny conditions.