Sorry for the delay i was coaching. Ok, Purigen is a granular substance, it comes in bottles and they sell bags for it as well. Depending on the size of your tank.
Amazon.com: Seachem Purigen 250mL: Kitchen & Dining
you can buy at alot of online stores i just wanted to give you an idea of what it is. Some of them come with a bag, others don't. I bought two bags and one 250ml bottle and it was enough to clear up my 55gal. I'm sure you can get away with just using a stocking, but i have a habit of buying things so i bought the bag they recommended.