The CPR Aquatic Bak Pak Protein Skimmer Pre Skimmer Box works with the CPR Aquatics Bak-Pak 2R+ Reef Ready Protein Skimmer and the CPR Aquatics Cyclone Bak-Pak 2 Bio-Filter Protein Skimmer. If used with the Bak-Pak Dual Pak Protein Skimmer you will need one Pre Skimmer box for each of the two pumps. The Pre Skimmer will help ensure you get the dirtiest water off the surface. The Box attaches to the powerhead.