One purpose of the fuge is to remove "bad" stuff from the water, such as Nitrites/nitrates/phosphates - that's what the macro algae in the fuge does. If you are pumping 10x turnover or more through your 46g tank, that means 460 GPH through your 20g fuge/sump = 23x turnover and probably much more if you just look at the fuge volume. At this flow rate you won't be able to keep sand in your fuge, much less algae ;)

If you are turning over less water through your tank (say 5x) and adding flow to the tank via powerheads, etc., that is still a bunch of flow for a fuge. You really want a low flow (5x or less of the fuge volume) flowing through it in order to give it time to "work" - so if your fuge is say 10g, you want to pump 50 gph through it, maybe even up to 100 gph, but not more.

Regarding Ehiem pumps, that all depends on the target flow rate you are trying to achieve from the sump to your main tank? They have 158, 317, & 602 GPH models, any of which may be appropriate based on your target flow?

Just taking a section of your sump, with full water flow, and "making" it a fuge won't accomplish the desired resuly (IMO). Somehow you need to flow most of the sump flow around the fuge - browse through fuge posts and look at various designs for ideas.

Hope this helps?

Oh, and I am sure Dan will response if you try again. Or many LFSs carry his overflow products that he mass produces - check Texas Tropical. (Just noticed you are Austin - scratch the TX Tropical suggestion).