im not as cautious as some of the others, i still have a couple of working units in play from time to time. carbon i dont change out as often as the americans seem to, why if your carbon is good quality & still appears to be working

. like anything the carbon along with the canister contents gets a good washout in the changeout water, do it not, and then the troubles can occur.
As for bactera populating the unit, who cares, surface area comparisons in a simple external canister dont even compare to the rest of the
sh@t we have running, so if people want to worry about that, let em i say!