Well, it all really depends on how far you need to pump the water and whether or not you will have any other sources of flow in your tank. If you want everything to be done with one pump (not the best idea) then you'd obviously need a larger pump. However, if you will have other sources of flow (be it from closed loops or powerheads) then you can get by with a smaller pump.
Personally, on your 33G (assuming that you are using a standard 30" tall stand, using a SCWD and the manifold as described above), I would use either a Mag 9.5 or Mag 12 if you needed to run the pump in the sump, or a nice small pressure rated external pump like a Little Giant 4MDQ-SC, or CSL T3. If you really want to splurge and spend $$$ on a nice pressure rated pump, you could do an Iwaki 40RLT. The reason I oversized the pumps a bit was from the fact that the SCWD and all those elbows and tees add a lot of headpressure to the pump.
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