A .12 - .18 size tuned pipe should be fine. I believe a .15 size tuned pipe is no different than a .18 size.

Are you adjusting the LSN at all? To me it sounds like the LSN is set way too rich and you must turn the HSN in to compensate for this. Check to make sure before playing around with the LSN as I am not there to see what is actually wrong.

I always use the minimal amount of pressure line to my tank. Usually about 2 inches.

Ryan