Go in and pull it apart, shake it and spread it around a couple/few times a week. It's getting too much detrius and debris built up in it. How is the flow on it? If you can get it to roll it would be perfect although I can't. Sometimes people get the turnover too slow in there too. For the volume that the cheato is in, shoot for about 20+X turnover (not the tank or whole fuge, just the compartment the cheato is in).