4 pcs Hydronix RO Reverse Osmosis pre filters. All filters are Premium NSF Certified Hydronix. This set is used as a mid year replacement set (depending on water usage and quality) where a replacement membrane is not needed. Replacement membrane is usually replaced once a year while your pre and post filters can be changed 2-4 times per year (depending on water usage and quality). This universal set s compatible with all major RO Systems with standard size housing. Stage 1 is a 5 micron Polypropylene (Poly Spun) Sediment Filter, Stage 2 is a UDF Coconut Shell Carbon filter, Stage 3 is a 5 micron Coconut Carbon Block filter, Stage 5 is a GAC Polishing Inline filter with 1/4" NPT Ports. This set is compatible with APEC, iSpring, Express Water, US Water RO Systems, US Water Galaxy, US Water Aquapurion, Watts 5 stage systems, Purevalue, Nelson, Isopure, Tier1, Hydronix, Max Water Systems, AQUARIUM, Titan Water Pro, Home Pure, Premier Water System, Blue Wave, and all standard 5 stage RO Systems. Also Compatible With:, 5EZ50, GTS-550S, NRO05-50, 200-GX-5050, ISO-RO5, GTS-550, Tier1 RO5-WF, RO550-W, W-525, and Many More.