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I`ve never had a canister but push come to shove you can get those mesh holders and put it in there and put in sump or hide in tank. The ideal thing is to find out where they are coming from.
I have a HOB filter and when the carbon is spend (after abotu 2 weeks) I will cut the filter open and dump the carbon. Then I refill with carbon and phosphate remover and seal. I put it back i nthe filter and let it run. It definitly helps with PO4. I am not trying a mesh bag in the HOB filter.