Skimmer before or after Biofilter

My tank is an AV 620 which means for those metric heads its 2ft wide, has an inbuilt filter that is hopeless and has no skimmer.

So I'm playing around with SolidWorks designing a new hang on filter to this set up, And I was wondering. Which is better. Skimming the water before the biofilter so the bio cycle has less proteins to convert to nitrate or after for some reason I know not yet.

 

I really don't see where the placement of the skimmer would make any difference at all. Its all the same water with in the same system.. Now if you pre-filtered the water before entering the Bio tower then that would help the most. The skimmer will remove the DOC's from the water that passes past it no matter what there in the passing water is or where it is in the sump..

 

Before is better, but the flow through the biofilter is much higher than the flow through a skimmer so it is tough to arrange. Normally skimmers either run independent of the bio or are located in the sump. The skimmer on my 210 is in the sump after the bio. The skimmer on my 300/90 gets about 1/2 of it's feed water after the bio. Both work fine.