The Laguna PT578 is a filter media bag that is perfectly sized to fit the following:
The Laguna PT578 Pond/Aquarium Water Filter Media Bag is made of mesh material that is very permeable. This aquarium filter bag can hold most loose filter media, and makes loading media in filter chambers quick and easy. This PT578 filter bag measures 8 1/2 inches x 14 inches.
The Laguna PT578 specifications: