canister filters are great nitrate factories. Totally work in freshwater, but not salt. You could take all the media out and put in LR if you want. But the media just collects detrius and converts it into nitrates. Different matter if you cleaned it every 3 days, but who wants to do that?
You could use a HOB filter and clean the media there frequently. (Much easier and I know of people that do this)
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