I ran both in same reactor. Layer of carbon......gfo.....then more carbon. I believe others here do the same as thats where I got the idea. I ran a slow flow rate through it.
One so called problem with the combination, is the gfo usually last longer than the carbon, thus requires less replacement. I just a small amount of gfo everytime.
Never seen much of a ph drop. As mentioned keep an eye on your alk if using lots of gfo as it can lower the alkalinity.
Should mention, like several others, I just run my carbon and gfo in separate filter bags in the sump or powerfilter if no sump. Found it a pita to change media in the reactors.
