I run 2 2lf reactors back to back using a eheim 200gph pump. I put the GFO reactor first in line, GFO at bottom of reactor and adjust flow to a light surface tumble. This feeds into the carbon reactor, Carbon is held to the top of its own reactor with foam inserts and held firmly enough to keep it from moving. The pads keep it in place, I suppose I could run the carbon at the bottom but its just the way I do it. Carbon can take as much flow as you can throw at it (within reason) and at the rate of a slight tumble of the GFO it seems fine. Have to pretty much remember unless you run a huge amount of carbon it pretty much is exhausted within a few days anyways. You dont want to over strip the water with the carbon unless there is something specific your trying to remove (medication/poison) I run a cup of each, and change carbon monthly and GFO every 6 weeks.
Hope this helps
