In my opinion, which in no way is expert or professional:
The only time you need activated carbon in a tank:
1) In a cycling tank: It helps absorb ammonia and nitrites, thus reducing spikes. Once biological filtration has been fully established, carbon is unnecessary unless for some of the reasons stated below. Frequent small volume water changes would also benefit in a cycling tank to reduce spikes.
2) If your water is high in heavy metals: It helps absorb a bit of the iron and other heavy metals from your tank. Tap water conditioner should also help reduce these levels by binding them into inactive states, but, especially in the case of iron, special filtration may be needed for inverts and plants.
3) If your water is high in chlorine: Tap water conditioner will also help, but carbon will absorb chlorine as well
4) If you use well water: And it has not been filter before it enters your house to your tap. Use tap water condtioner, this water is usually high in iron and copper from pipes.
5) After medicating a sick tank: Carbon will absorb the medication and remove it from the water
6) If anything chemical has entered your tank that you don't want in it: Activated carbon is what posion patients are made to swallow after ingesting a toxic substance. If you have a tank in an office or someplace professional cleaners are using their industrial strength stuff, you may want carbon in your tank to help remove whatever incidental volitiles that may end up in the tank. I'm not talking pour the whole bottle of glass cleaner in the tank, that would need a water change to rectify.
7) Funky smells or colors: If the water turns funky, carbon can help absorb some of the damage, but water changes would be better.
If you have a fully established tank then the bulk of your filtration should be done from whatever biological filtration you have set up. I prefer Marineland Biowheels because they seem to contain very good bacterial cultures and allow the bacteria to preform aerobic metabolism by the constant rotation and exposure to O2. Some people have achieved the same thing with AquaClears by only using sponge pads in their filters. Whispers have the biobag in which the carbon can be replaced but the bag itself saved. IMO: Marineland Biowheels provide more surface area for bacteria as well as a better aerobic conditions for bacterial growth, thus providing better biological filtration.
Fish stocking levels are proportional to the amount of biological, not chemical, filtration you have on your tank. Chemical filtration aka carbon, gets used up quickly and can be costly to repeatedly replace (the Marinland 330 replacements cost $30 for a three pack where I work). Since I don't have that type of money to be spending every year, I simply wash off the cartridges (two of them are a year old) to get macrojunk off them, and put them back. Is the carbon still active, heck no, but I use the floss part simply for mechanical filtration and let the biowheels and water changes take care of water quality. I've had two biowheels on one of my 330s cultured for a year now, and I can't report any water quality problems at this time. I water change twice a week though, so it just could be my O/C behavior that keeps the tanks so clear *shrug*. I check every day to make sure I have good rotation and adjust the biowheels accordingly.
~~Colesea
