I have been running a Mag 5 on my 90 gallon reef and have recently decided to upgrade to a mag 18. I was looking at the instruction manual on the Danner website and it says to use at least 1 1/2" pvc for the outlet. So, I figured "why not go for the 2"... ", until today when I went to look at the 2" ball valve and other parts at Home Depot. I was wondering if anyone else has used 2" PVC for a pump this size and if it would really make a difference over the 1 1/2"? It just seems like it would take up alot of space and might just be overkill, but I wouldn't mind running it with 2" if it really made a difference. I also plan to run this pump externally. Anyone have any good or bad experiences with this as an external pump?