Rot generally starts with mushy purple or black spots towards the base of the plant before spreading all over. The leaves may also start rapidly dropping off, or the center of the plant might fall out. A rotting air plant is generally a sign of excess water or humidity. Air plants grow in dry environments, and while they need to be watered deeply and prefer air humidity to sit around 65%, they don’t like to remain wet.
You should be watering your air plant only every week or two, depending on the species. Fill your sink with water until it’s high enough to submerge the plants, and soak your air plants for approximately half an hour. Place them upside down and let them dry for an hour or more to allow water trapped between leaves to drain out in order to eliminate any possibility of trapped moisture. This is particularly true of air plants in terrariums, as the lack of air circulation will cause moisture to get trapped in the center, quickening the rot.
It is also important that you don’t place air plants on the soil of other plants or on a pebble tray, as the constant contact with a moist surface can cause rot.
Natural Life Cycle
It is natural in many cases for older lower leaves on an air plant to turn dark and fall off. Air plant leaves that fall naturally will turn brown or yellow, but stay dry.