If you use liquid fert. the carbon will absorb whatever you add to the tank. Personally I only use carbon bag if I had to treat a tank and than remove that treatment or if the water looking murky, as will as very six months as a preventive measure, from the tap water.. Carbon filter will only absorb as much chemicals as it can hold, and than start leaching back into a water, like a sponge. I use a good carbon about 3 to 4 cups poured in a bag and just place that in the filter for the month, when I do my monthly filter break downs. I find it cheaper to buy loose carbon, than buying premade pads.