many thanks for the link, still not convinced if carbon is a good or bad thing or if it's absolutely necessary. i have a fluval 2 in a 60 litre tank in which live 10 danios, i'm thinking of taking out the carbon media and replacing it with a second "regular" sponge.
Most of us don't use carbon on a regular basis. With 10 danios in that size of tank, extra biological media (filter sponge) would be of more benefit. It's a good idea to keep a spare carbon pad aside in case you need to medicate at any point as it is useful for removing the last traces of medicines after treatment is complete.
I put carbon in my filter for maybe three weeks after medicating as it absorbs the medicine from the water to help protect filter media. I say it's okay for that and I always rinse mine out before adding it so the dust doesn't get in the tank