These cartridges are essential if you have a Fluval G6 filter, but the manufacturer recommends replacing them every few weeks! It will cost you almost $600 a year! That's more than the cost of the filter itself! I change them about every 4 1/2 weeks and my 50 gallon tank and fish seem to be doing fine. I've read of other people keeping the cartridge case and filling it with a different product, but haven't tried it yet. I only give it 3 stars because of the high price.
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