This sounds like and HOB filter(Hang on back) A Cannister filter is more bucket shaped and normallly stays under the tank. I only keep the filter cartridges in my HOB filters, but when I do a water change, I swish them around in water I have removed from the tank and return them to the filter. I rarely ever replace mine and if I do, I only replace one of the 2 in the filter. Hope this helps.
Also...the bacteria doesn't only live in the filter, it also lives in the substrate of your tank. Many people report a "mini cycle" after replacing all the substrate.
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