Pen Plax TDLBX Large Tank Divider for Aquariums, 11-3/8" x 11-3/8" LARGE SIZE: perfect for 15-20 gallon aquariums. Other sizes available, sold separately. TURN ONE TANK INTO TWO: allows for the separation of fish without major modifications to your tank. PERFORATED POLYMER SCREEN: the divider wont restrict circulation, filtration, or heating. SEPARATE YOUR FISH: perfect for separating aggressive fish or fry. CAN BE TRIMMED: for non-standard sized tanks, the polymer screen can be trimmed to size.