I am running the BRS dual reactor, both canisters half full of carbon and GFO mixed 50/50. Every week as part of my routine I change out the lead canister, rotate the lag canister to the lead position, and put the newly filled canister in the lag position. So basically each one gets replaced every 2 weeks. Carbon used for polishing water in an aquarium is more than likely shot after 4 or 5 days, but becomes an effective biofilter. My thinking in the way that I rotate mine out is that by placing the fresh canister in lag behind the lead that has been in there for a week already is that bacteria quickly populates the fresh lag canister. I then flush about 5 gallons through the canisters to remove GFO fines, and that's my weekly water change.