Steve, depending on the length of run and the depth of the diffusers, you could see a pretty good drop in psi. Maybe an extreme case, but when I swapped 3/8" for 1", then necked it down to 1/2" going into the pond at a customers I saw a 6 psi drop in back pressure.

www.hoosierpondpros.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).