Kay:

Cecil touched on it briefly. What type of diffuser do you have, an airstone or a membrane type? IIRC, airstones need to be cleaned periodically, and the membranes should be cleaned as well, I've just turned them off and on a couple times and that seems to work (the membrane type). Does your line running to the diffuser have a pressure gauge on it? If so, what's it read?

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).