What type of diffusers are they (ie, rubber membrane, stone)?

I installed my system around the same time and it's been down for about the same length of time. You've got me curious, to say the least.

.10 surface acre pond, 10.5 foot deep. SW LA. The epitome of a mutt pond. BG, LMB, GSF, RES, BH, Warmouth, Longear Sunfish, Gambusia,Mud Minnows, Crappie, and now shiners!!...I subscribe!!