I'd perfer a xp3 on a 55, but the xp2 would do it. I have a 180g that had one xp4 on it and it wasn't enough for me, so I added a second xp4.
For the record, xp2 = 150gph, xp4 = 200gph. The pumps aren't that much different, the filter volume is what changes the most.
Are we having the same...