Coralife Pure Flo II Reverse Osmosis Carbon Block Pre-Filter CartridgeChlorine and chloramines will damage your R/O membrane and the use of a carbon pre-filter can thus improve its functional life. This filter should be positioned between the micron cartridge and the TFC membrane. Additionally, these carbon pre-filters also reduce the levels of some SOCs (soluble organic compounds) like pesticides, dioxins and VOCs (volatile organic compounds like chloroform and petrochemicals).Maintenance/Lifespan: Carbon pre-filters should be replaced every 3,000 gallons or 6 months (whichever occurs first). Depending on the quality of your water source, you may need to replace these pre-filters as often as once per month. If not replaced - certain elements (such as chlorine) will become present in your purified water and your membrane cartridge will also become damaged.