The Fluval A237 is the correct replacement for the following canister filters:
The Fluval 306/406 Bio-Foam Aquarium Water Filter is a includes two replacement aquarium filters for the Fluval model 304, 305, 306, 404, 405, and 406 canister filters. The Aquarium Bio-Foam pieces promote efficient biological filtration in aquarium water and trap small particles and debris. This aquarium foam filter also provides an additional layer of mechanical filtration and is best suited for freshwater and marine environments.
Fluval 306/406 Bio-Foam Aquarium Water Filter specifications: