I do not understand why this skimmer is not listed in the Red Sea website skimmers section. I just mounted one in a 20-gallon rectangular reef tank. Fits great. Installed at noon today. Now 6:00 PM and I have quality skim and buildup in the skimmer stack. Takes up about 40% of the AIO filter area. I also have a Red Sea 20 gal cube and have loved its performance in that setup. Seems to be available on European reef supply sites but only Saltwater Aquarium..com lists it here.