Ted, thanks for the suggestions. I have noticed that originally I was running about 10 psi but it has gradually risen over the last several years. I am familiar with various ways that air stones can clog and since they aren't expensive I just replace them every other year. This last time I did wonder why the pressure really didn't drop with the new stones and I wondered if something could be wrong with the check valve. You've given me some things to look for. Since it is winter now I may try just running some muriatic acid through the line to see if the problem is crud in the check valve. If this helps fine. If not I'll pull the thing up come spring. Some day I'm going to start a thread just to discuss the difference between air stones and diaphragm diffusers and I bet you know a few things about that. Should probably be a new thread.

Gotta get back to fishin!