I've been running my pond aerator practically 24/7 for nearly four years. This season I noticed that the bubbles were barely coming to the surface anymore. I had heard that the porous stones would need to be replaced, so I ordered and replace those, which made no difference. I did a little more research online and found this kit. I installed the new diaphragms, and airflow was much, much better. The downside is that now the unit is a little less quiet. It was never silent, but now there's a slightly louder hum.
Between the cost of the new air stones and the diaphragm kit, it was almost enough to just purchase a whole new unit, which in hindsight I wish I'd done. But this was an ok fix.