MP20: 500gph. MP40: 1000gph. Get the MP20. You can always upgrade later. Two MP40's on a 30B would blow your LPS into next week. I have one MP20 on a 3' 65, and my coral focus is LPS. The duncans, acans, scoly's & bubble mushrooms prefer the gentle back & forth linear flow, and the prop is big enough that you'll be moving a wall of water & still desedimenting in order to keep the crap in suspension. IMHO.