Bestway Filter Cartridge (Twin Pack) contains two type I filer cartridges, which are compatible with Bestway 300/330 gal/h filter pump systems. When an above ground pool is in regular use you will soon notice the filter pump running less efficiently and water becoming dirtier. This is because the filter cartridge is clogged with dirt and debris. There are only so many times you can clean it before it stops filtering grime and producing clean, clear water. It's recommended that you change your filter cartridge at least every two weeks during periods of prolonged use.