I use Centaur carbon in my system, and it has worked well for 10+ years. Your retention tank sounds small, though -- you need a minimum contact time for the chlorine to completely oxidize the ferrous iron to ferric, and that is dependent on the flow rate and tank size. My tank is 125 gallons, and we don't have a particularly high flow rate -- maybe 10gpm peak, giving a ~12min contact time. One source recommends:
Amount of chlorine required to kill bacteria and oxidize iron, manganese and hydrogen sulfide:
and in our case, it seems to work as desired. I use a spun filter downstream from the contact tank which verifies that the iron is being oxidized, and keeps the big pieces out of the carbon. After the carbon, the water is pure but still has residual hardness, which the softener takes care of. Overall, we're very happy with it. The only real problem is that with no residual chlorine in the water, we see objectional mold growth under the rim of the commodes. So, I'm thinking about re-chlorinating downstream of the softener. My plumbing will allow selective delivery of the newly-chlorinated water to any desired fixture(s).