From the Fluval Manufacturer Web site:
The AC Series takes power filtration to the next level. Built on a best-selling platform with a 40-year reputation for exceptional quality, reliability, and convenience, this series has become a standout choice that aquarium enthusiasts know and trust. Along with its proven multi-stage cleaning, CycleGuard beneficial bacteria support, and patented refiltration system, the latest clip-on generation sets a new standard for cleaning performance, media efficiency, quiet operation, and durability with several notable upgrades. Experience the unmatched cleaning power of Fluval AC Series!