These things are expensive, but if you need them and don't want to buy the whole filter, this is your only option. . You can skip the black cage and buy a slightly thicker sponge and place it in the same spot. The cage is meant to be easier to clean, or rather foolproof for less experienced fish breeders (like me). You will not regret it. Cory from Aquarium Co-OP explains it best.
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