Ted, my pump is designed to operate up to 15 psi according to Thomas. I have a 15 psi relief valve on the outlet manifold. You are correct that the 4 stones aren't really meant to take 5 cfm. If I remember correctly my pump is rated about 5 cfm at 10 psi. So I should be putting out close to that. I have not installed more stones because in the summer I run two diffusers off this pump and then the stones only see about 2.5 cfm. I didn't think it would be good to run 6 stones half the year at only 2.5 cfm. Thought they might clog faster. Any thoughts on that?

Gotta get back to fishin!