What model of filter?
I have a Fluval 304 (which is a canister filter). It comes with 4 foam inserts and they just clip into a vertically sliding tray that sits beside the media boxes (the way that EMS503 described).
I also have a couple of the Fluval 4 (which are internal filters). The sponge filters are cylindrical and just slide over the intake tube. The foam inserts are like sleeves and they just slide over the intake tube.
I hope that helps.
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