Instantly converts one aquarium into two - allows unrestricted filtration. The One and Only... Another Unique Penn-Plax original! Four Sizes Available: *Medium (Fits 10 gal. Tank) - 11-3/8" x 9-5/8" *Large (Fits 15 - 20 gal. Tank) - 11-3/8" x 11-3/8" *Extra-Large (Fits 20 gal. Tank) - 11-3/8" x 15-1/2" *Super (Fits 29-55 gal. Tank) - 11-3/8" x 17-1/2"