I use activated carbon in all my tanks (past & present) for chemical filtration. When I eventually get our xP3 set up, I will be adding carbon to the spare compartment. Carbon is particularily effective at removing certain chemicals, especially chloramine & chlorine, and has the side effect of keeping the water very clear. We will have some fish in our turtle tank and I prefer to have carbon in the filter with fish in the tank.
Here is a list of what carbon is effective at removing from aquarium water;
High to very good absorption
Arsenic, bleach, chloramine, chlorine, chromium, colors, dyes, gold, insecticide, odors, monochloramine, tin
Good to Moderate absorption
Acetic acid, cobalt, detergent, hydrogen sulfide, mercury, ozone, potassium, silver, soap, solvents, vinegar
Fair absorption
Copper, iron (not chelated), lead nickel, titanium, vanadium
Little or No absorption
Alkalinity, ammonia, barium, carbon dioxide, hardness, copper, manganese, nitrates, selenium, molybdenum, zinc
Optionally, the only other good possibility that I considerred (like Chris suggested) would be to move the biological rings up a level and put the pre-filter prior to the rings in the water flow. Since you already have the ceramic rings and the Bio-Chem Stars, the Fluval Pre-filter just seems like over-kill in the biological filtration department, so carbon probably is the way to go for that empty compartment in your xP3.
(Chris, I'm surprised that you do not use carbon, especially considering your 'obsession' with clear water...

....also interesting that you are using Fluval Pre-filter ceramic instead of Fluval Biomax ceramic, although I guess they essentially do provide similar biological filtration capability in the end. In any case I guess the xP1 does have some compartmental limitations....

)
1 RES, Spike (In new home with 1xPleco, 1xRed Zebra Cichlid, 1xYellow Lab Cichlid)
2 Cats, Blossom & Panda
2 Tropical tanks (plecos, platies, tetras)
2 Handsome Blue Bettas, Blueberry & ?
1 Bright Red Betta, Strawberry
1 Feeder guppy breeding tank