What is your goal of the refugium? Are you setting it up as a way to lower nutrients, or as an actual refugium for micro-flora and fauna?
Using fuges as a way to 'clean' your water is a relatively new notion in the world of fuges. The original idea of a refugium (and hence the name) was to provide a safe place for organisms that would be consumed/otherwise killed in the rest of your system a place to grow and reproduce, serving as a constant source of near microscopic animals to replenish what's being depleted from the display. Hence the lower flow, as you want a place where the smallest of animals and macro/micro algaes can grow without being washed away.
If your goal is for the fuge to 'clean' your water through the action of macroalgae consuming nutrients, chances are good that you're going to need a much, much larger refugium than you had originally planned, and there are far more efficient and effective set-ups/designs that than a fuge if this is your goal. A small corner of your sump is going to be functionally irrelevant to the overall nutrient regime of your entire system. Also, if the goal is the reduction of nutrients, you'll want the highest flow the plant life you place in that fuge can handle without getting damaged/blown away moving through the chamber, otherwise your fuge will simply be a place for detritus to collect, adding to the nutrient problem.
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