The Unicel C-4405 is a matched pair of replacement filter cartridges delivering 50 square feet of combined filtration for Pentair Rainbow Dynamic Series IV systems and Waterway skim filters. It is sold as a set of two because the system is designed around two cartridges working side by side — not because two is a convenient bulk quantity.
That distinction matters more than it might sound. These housings run a pair of compact cartridges in parallel rather than one large one, and the 50 square foot rating you see on the system — Rainbow designates it DSF-50 — describes what both cartridges deliver together. Each individual cartridge accounts for roughly half. If you have seen the equivalent aftermarket part listed as a “25” and wondered whether you were being sold something smaller, you were not: the number simply refers to one cartridge instead of the pair. Both figures describe the same hardware.
The practical consequence is that these two cartridges live and die together. Water entering the housing splits between them and takes whichever path offers less resistance. Put a fresh cartridge next to a used one and flow skews hard toward the new one, which then loads up at an accelerated rate while the old cartridge does progressively less work. The system never reaches its rated capacity, and the new cartridge wears out well ahead of schedule. Replacing as a set is not an upsell — it is how the design is meant to function.
Each cartridge measures a compact 4-15/16″ across by 6-5/8″ long, with open 2-1/8″ ends carrying molded lip seals — the raised sealing edge that presses against the housing and blocks unfiltered water from slipping around the outside of the media. Construction is Unicel’s standard: pleated polyester Reemay media over a solid molded core, with chemical-resistant end caps built to an exact OEM fit. Made in the USA by Meissner Manufacturing, with more than 45 years behind the Unicel name.
Pro Tip: Always install both cartridges at the same time, and never pair a new one with a cartridge that has been in service. It is tempting to swap only the one that looks worse, and it consistently backfires. Flow follows the path of least resistance straight to the clean cartridge, so the new one absorbs a disproportionate share of the load and clogs early while the old one coasts. You end up buying cartridges more often than if you had simply replaced both at once. This is also a good argument for keeping a spare set on hand.
Brand
UNICEL®
