Aqueon Betta Filter Cartridge Replacement One Size

Each pack contains 2 replacement cartridges.
For best results, replace cartridge once a month.
For small betta habitats, such as bowls and aquariums up to 3 gallons.
The replaceable carbon and sponge cartridge keep water cleaner and clearer.
To replace, simply remove the air pump, twist off the old filter, and twist the new one into place.