Best place for carbon

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I want to run carbon in my sump.

I put a cupful of carbon in a bag made of my wife's old panty hose and stuck it in between the sump baffles, it floated to the top.

Then I floated it in the sump, it seems that water would flow around the bag full of carbon rather than through it.

I'm considering just laying some loose carbon on top of an open tray on the floor of the sump.

Anyway I just want to maximize the contact time of carbon with water, what is the best way?