I would have to agree on the two #2's Thats exactly what i have currently installed in my 55g. Trust me, they provide plenty of flow to the entire tank. Plus, if you have something that creates too much flow for that tank, either you will have a sand storm in the tank, or you wont be able to have low flow corals. Also, you'd be surprised as to where the flow exceeds itself when theres rock in the tank. The flow bounces off the rocks and goes just about everywhere. I have one of my koralias pointed directly at one side of my rocks, and the other pointed at the front panel and everything is perfect. IMO
, two #2's work great in a 55g.