part of your filter. There is a biofilter in almost all filters which is basically a sponge that bacteria grows on. Some filters have a second "cartridge" that fits in behind the sponge. This cartridge contains little black rocks called activated carbon. This carbon will remove the medication from the water. This is a good thing once the fish are cured but untill then you don't want carbon in your filter.
I have two suggestions if you have carbon
1. buy another carbon bag and throw the old one away. once you finish treatment put in the new carbon bag.
2. If your carbon bag can be opened (many can but it's tricky figuring out how to open them) then you can buy a bucket of activated carbon. Pour out the carbon in the bag and rinse it off then put the empty bag back into your tank. Then once medication is complete remove the bag put in about the same amount of carbon that you took out, rinse the bag well, then put the bag back into your filter.
Carbon is only really necessary when you first set up a tank and after medicating. I keep empty carbon bags in my filters so I can put carbon in if some chemical get's dumped into the tank or if I have to medicate for something.
