Good - Single Canister - You can run
GFO or
GAC. While not ideal, it's also possible to run both in a single canister by placing the
GFO in the bottom and
GAC on the top (catches
GFO fines) separated by sponges. The downside is
GAC typically last much longer than
GFO. When used in a single canister, you're forced to change out both at the same time.
Better - Dual Canister - Allows both media to be run in their own canisters and changed out individually.
Best - Two individual Canisters - Same benefits as dual, but adds the ability to control the flow rates individually (you want
GFO to tumble, whereas the
GAC should have lower flow for longer contact time and to prevent tumbling and the creation of fines.)
Avoiding
GAC fines is important as studies suggest carbon fines contribute to Head and Lateral Line Erosion (HLLE) - also referred to as Lateral Line Disease (LLD.)