My tank is 3x3x2.5 and I am using eheim 1262. Still ok because I have lots of liverock. My liverocks acts as the biofilter instead of my sump. My sump has a little cheato farm, which is dying.. may be not enough nutrient already, CR to add some calcium to the system, pump for chiller, pump for skimmer feed, biohomes to house some bacteria, FR which I have not change for months ..
I only concentrate on internal water circulation. Probably got around 20kL/hr. May be not enough.