I am using carbon/sponge filters in two of my tanks and I wonder how the tank can cycle if the carbon absorbs all the chemicals and bacteria. Any ideas?

Carbon doesn't absorb all the chemicals and bacteria. It does almost nothing to ammonia and nitrites, which are the important things for cycling. It also does not absorb bacteria.

Stay away from ammo chips and nitrazorb in any tank you intend to cycle (unless you have specific problems you need them for, like ammonia in your tap water), they do interfere with cycling by absorbing ammonia/nitrites and starving the bacteria.

Thanks, I was obviously mistaken.

OK, if the carbon does not absorb chemicals, gases, bacteria, etc, what does it do? Why does it have to be replaced every 3-4 weeks?

Carbon absorbs some chemicals, ammonia, nitrite and nitrate just don't happen to be among them.

To be honest though, it doesn't tend to absorb anything of much importance in the day to day running of an aquarium, thats why many of us don't use it.

It removes medication and smells. It can only pick up a certain amount of stuff before it is "full" and needs to be replaced. The most pessimistic estimates put this point at about 3 days after you first put it in the filter, and the most optimistic put it at about 2 weeks. 99% of fishkeepers will agree that the 3-4 weeks suggestion on many filter boxes is dumb as the carbon has been long since worn out.

carbon absorbs pretty much any impurities that are found in aquariums (and air).actually, carbon does absorb certain amounts of ammonia, nitrites and nitrates. it just doesnt absorb quite enough to make any difference in the cycling of new tanks. i dont know this as firsthand experience, its just something that ive read on a few different pages online. so, take my post with a grain of salt.

and to back up ceridwens suggestion, i havent used carbon in any of my tanks for more than 6 months and everything is going fine. i do, however, use carbon to remove meds from tanks that i treat.

paul

i replace the carbon every two weeks in my Fluval 304. I look at it as the oil filter and water changes being the oil. both are important to change on a regular basis.

That's interesting guys, that sorted things out - I was somewhat confused.