This is a pack of 3 replacement water filter for the Isopure Water 4 Stage Reef-Aquarium Deionized Reverse Osmosis Water Filter System. Membrane is not included. Set includes 1x 5 micron sediment, 1x 5 micron carbon block, and 1x color changing DI filter. Replace pre- filters every 6-12 months and di filter life is 5,000 ppm. Reverse osmosis removes a large majority of TDS (total dissolved solids) but not enough of it. TDS can be lowered to extremely low levels by using a DI (deionization) filter. With our filters, you will typically get ZERO to near ZERO TDS Additional Details ------------------------------ Bullet point: Set includes 1x 5 micron sediment 1x 5 micron carbon block and 1x color changing DI filter.|Replace pre- filters every 6-12 months and di filter life is 5 000 ppm.|Reverse osmosis removes a large majority of TDS (total dissolved solids) but not enough of it. TDS can be lowered to extremely low levels by using a DI (deionization) filter. With our filters you will typically get ZERO to near ZERO TDS