Charcoal is a very yes/no with many aquarists.
The facts are that it will remove medications, and some/mamy other additives when added to the tank.
It has been calculated if used to remove/clean a tank after a medical or other treatments the charcoal "MUST" be removed immediatly.
Another concern is that it actually does not last very long at all (depending on its usage purpose)this can be far less than four weeks, and should be removed.

Stating all that
Yes it has a very good purpose.
No if left in a tank too long it becomes useless and can cause other bacterial problem concerns.
To answer the Bacterial concerns I will leave that up to the highly qualified people not just the aquarium keeper.

Keith

I used to use the carbon that came on my filter's cartridges, but I got wise and quit using those. After reading and talking to an LFS worker who actually knows what he's doing, I found that carbon is pretty much useless. All you need is that white spongy stuff they sell in sheets, and then something for bacteria to grow on. Buying media in sheets and cutting it to size is much, much cheaper, and you can clean it and reuse it until it gets too dirty. Carbon is good for removing medications, metals, and discoloration. I think it's good to have some on hand in case you need it, but why waste it?

I don't know if charcoal is any different from the sheet and pellet forms of carbon, but I haven't heard of either of those two doing any harm to a tank. I once used the same cartridge for two months without any problems.

Carbon is like any other media. Use it properly and it's beneficial. Don't, and it either does no good at all, or can actually create problems.

It's true that it will negate medications, but the properties that make it unsuitbale for a medicated tank are what also makes it useful for removing them, and other impurities from water.

Well, it depends on what you mean be "filter media". Bio-media is FAR more important than carbon and it is the key to keeping your aquarium healthy. An example of good bio-media is the ceramic bio-ball or tubes. If there is any space in your filter and you don't already have this, get stuffing, because this is what keeps the nitrogen cycle stable and acts as a "house" for beneficial bacteria. That way you can easilly change the cartridge when its gross and know longer strains water from waste. Cartridges can hold some beneficial bacteria, but should not be depended on for that sole purpose. Though you shouldn't be replacing them more than once every few months, if you don't have bio-media, your cycle will crash when you exchange or clean the cartridge with tap water.

So, in a typical HOB style filter, there are three types of filtration: biological (bio-media..sometimes this needs to be added to the filter yourself. Never change it.), physical (cartridges and sponges strain waste from water), and chemical (carbon, etc). Most chemical filtration is not really worth using all the time. It claims to keep water clear and smelling good, but if you have hazy or smelly water in the first place, you should be fixing what is wrong, not trying to cover it up. Carbon can go bad, so always check how it smells before putting it in the filter. If it smells like sulfer, dump it. Also, you must exchange the carbon every 2 to 3 weeks. This is very inconvenient if its already packed into the cartridge and you don't have biomedia to hold a cycle. Another example of chemical filtration is zeolite. In the typical aquarium that is well taken care of and not over stocked, an aged filter with lots of undisturbed bio-media will never have ammonia. Adding zeo-lite to a filter that is cycled or you plan to cycle will disrupt everything.

So in my opinion, carbon and zeo-lite are better off not being used on a daily basis. I think the only real good reason to use carbon is to remove medication in an aquarium. Zeo-light can be used in a filter of a tank that you do not want to cycle, whether its too small to keep safely stable or its just a temporary tank (like quarentining new fish) but be sure to let it sit in salty water every couple weeks to leach out ammonia and re-use the chemical. Never add salt to an aquarium with zeo-lite because it will make it expell all the ammonia into the water.

Hope that helps.

~Lauren