If you are refering to the KENT MARINE brand/type,you have to leave it directly in the flow of water for at least two months before it starts to work.Then you must replace one third of it every month.Then after all this time and work,it only removes 20% of the total reading. Example(if your reading is 100ppm,it will lower it to 80ppm).Thats alot of time,money,and work for 20ppm. I tryed it,and quite after a month.I finnally got mine down by installing a DIY algae refugium,and replacing the CC with live sand.And reducing the number of fish in the setup.