in essence, yes but it can be done.
from research I've done I'm not sure it has to do with working at full potential. I believe it has more to do with the grain size. If I'm not mistaken, carbon has a finer grain size compared to gfo so it needs a lower flow rate so it does not get pushed back up the column of water and come out into the tank. Correct me though if I'm wrong though
. Performance wise though I believe that's all up to the amount of time a specific amount of water remains in contact with the gfo or carbon. So say you have it filled with gfo at a rate of 200gph and the same for carbon it should be about the same. However as stated above the grain size and density of the material would be more of a factor that would cause a change. However, if you were to mix them together, and maybe have a cap of gfo at the top, it may be do-able.
Of course there is always the limitations on how much media can be placed in it but putting maybe half carbon on the bottom seperated by a sponge disc then a layer of gfo, that may be a little better. Take the poly reactors for instance, what in there build quality internally is so different from the phosban reactor that allows them hold 2 seperate media types? Not a darn thing.
I will agree running 2 seperate reactors for each application is the most effective way simple because it exposes more water to an increased amount of carbon or gfo instead of a smaller amount of both in 1 reactor.
This information is based off reasearch of how they work and not from personal experience as of yet so if there is something missing or incorrect then I suppose it should be corrected or stated!
