Replacement cartridge for Nature2 N2 Series. Model N2CN25, N2CN35, N2CW35 SwimPure Cartridges fit inside of your Hayward cartridge filter. No purifier unit needed, these long, very effective Nature2 cartridges were once sold by Hayward under the name SwimClear. They drop inside of your filter cartridge. Sold in either 3 or 4" diameters, with lengths from 12" to 18" long.