There are 6 cartridges, so less than $1 each (assuming the price is less than $6). A unit can last more than a month if you use an additional filter system (e.g. a Purigen bag) inside the Penguin HOB filter. A cartridge easily lasts 2.5 months for me, as I have an aquarium with 2 filters (internal and HOB with purigen inside). My aquarium is overcrowded but the water is crystal clear thanks to both filters working together. I decided to buy a Penguin bio wheel filter (100 gph) based on the price of the filter and cartridges together (less than $10 for the filter and less than $6 for the cartridges) and some good reviews. My suggestion: wait until the price of cartridges goes down before buying. The price for size A (6 units) should be around $5.30, but sometimes you can find almost $7. There is room in the filter for Purigen bags (100ml) if you run the Biorad at 100gph, so a cartridge can last up to 3 months when used with a recycled Purigen bag (I recycle my Purigen bag every month, 3 days). after 50% aquarium water change) I do not regret the purchase.
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