The Reef Octopus Regal 250-EXT Recirculating Protein Skimmer takes the 300-635 gallon rated Regal 250 platform external. Water recirculates through the VarioS-6S DC pump outside the sump, extending air/water contact time for drier, more concentrated skimmate than the equivalent in-sump body typically produces. Bioload capacity: 300 gallons (heavy), 500 gallons (medium), 635 gallons (light), excluding sump Setup: external, recirculating Pump: VarioS-6S variable-speed pinwheel pump, 36V, 5 power settings Control features: float switch input, 15/60-minute feed-pause modes, 0-10V third-party controller input The recirculating design costs more floor space next to the sump but is the pick for hobbyists prioritizing skim quality over compactness on a 300+ gallon system.Compatible replacement parts: VarioS-6S pump kits sized for the Regal 250 series.