They are horrible. They cant maintain a siphon on their own and require a vacuum pump to do so. If the pump fails, clogs, or the air nipple clogs, you have a problem. In addition, you cant take it apart and cleaning the inside of it is a real PITA. A quality U Tube based overflow is MUCH more reliable. Get a Lifereef overflow and you will not have a problem. They are pretty much bullet proof.