its been a while so just to check, if I bought the little fishies reactor or any other all that i'd need is an extra sponge pad to separate the gfo and carbon to make it work. Its only for my 20 gallon nano.
Mixing gfo, which should tumble, with carbon, which should not is problematic.
Then the fact that they get exhausted at different rates even more so.
With that said imo would not use little fishes reactor for gfo. Most gfo media is iron based and little fishes reactor is known and prone to leak some of the media which will then lead to it finding its way onto pump magnets.