Order of Reactors & Contents

Carbon can't tumble,it will reduce itself to powder. That is what the sponges are for, to hold it in place. Flow does not matter on carbon, slow or fast does not change it's effectiveness.

You want the carbon to catch and dust coming off the phosban. Some GFO needs to tumble, not sure about yours. So you tumble the GFO first, and hold the carbon second to act as a filter. Flow is determined by the tumbling GFO, but if phosban does not need to tumble, then just go by what ever flow it needs.

You put in what ever the directions call for your system size.

 

You will find they both need different flow rates the carbon should have about twice if not three times the flow as the Ferric oxide , I would not series them in line myself

 

I wouldn't either.

EDIT: BTW, you can still run both off of one powerhead. TLF sells "T" fittings and ball valves for the tubing.