Carbon keeps the water sparkling clean, same as in drinking water purifiers, and comes already in the store-bought filter cartridges and you can add it to your canister filters. You don't have to use it unless you've put medicine in your tank, your fish are now cured and you want to remove the medicine chemicals. If you do put it inside your canister filter, you need to know that it absorbs chemicals in the water and retains it in the charcoal. After 2 months, it is used up and can reintroduce the chemicals back into your tank, so change it every 2 months.
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