Yes you can
You can add filter wool, or biological filtration media like biohomme/ seachem matrix/ceramic rings
Gex ss is really small. The larger the filter capacity, the better. What more that you've a small tank, which is more tricky to maintain due to the small volume.
My tank is 18L and I've a Gex Slim M and another hof that's the size of a Gex Slim S on it.
I've matrix in one and filter wool/seachem carbon in the other
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