Junior. Here is a 4-pack, but ultimatley you save money by buying the larger box.
The small is for whispers' small 1-3 gallon filters.
FYI though, Big-Als seems to be behind with the times. Whisper now uses a
small, medium, large setup. So, the next time you look for new cartridges you may also have to look for medium(which is the same thing as Junior).
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