Carbon dosing a saltwater tank is the process of adding carbon to the water in order to remove impurities. Carbon is a natural substance that is highly effective at absorbing toxins and impurities, making it an ideal choice for purifying saltwater. The most common method of carbon dosing a saltwater tank is to add granulated activated carbon (GAC) to the filter media. This will allow the carbon to come into contact with the water and remove impurities as the water passes through the filter.