Carbon life can vary depending on chemicals which the carbon is absorbing, the quality of the carbon, and ect. In addition carbon might only be truly effective for a few days (or just a day). Still, the general rule of thumb is normally between 2 and the absolute max of 4 weeks (rising the bags out (in salt water) between changes also helps alot). Carbon can/will leach stuff back into the tank if one does not change it out. I would personally buy a reusable media bag (just the nylon bag) and a bulk order of carbon media... those pre-fab canister replacement bags are expensive
As for the ceramic rings, if you have a high bioload and an established tank (aka you have been successful with the setup) removing the ring "whole hog" could risk a crash if they are serving as a a primary biofilter. Still, if you are intent to use the canister filter it can be beneficial with proper maintenance. The ceramic rings should be rinsed out in salt water every month to try to remove any settled detritus. I would change out 1/2 of the ceramic rings every 6 months anyways. The rings works on a similar/same principle as bioballs. While these media (including using live rock rubble) are notorious in their reputation as nitrate traps and producers, this is often as a result of improper maintenance or filter setup. Still, just make sure you rinse and/or replace that foam block regularly... aka rinse the foam block out every 3-7 days (the more the better) and replace the block every 2-3 months... mostly to keep detritus out of the carbon and ceramic ring area.
Last edited by FutureDoc; 04-09-2010 at 12:24 AM..