Hello Come Bash You (what a strange name...),
If you are using a sponge filter driven by air pump, then there's no need for a surface skimmer. The air bubbles breaking on the surface will ensure there is virtually no scum on the surface.
Traditionally surface skimmer performs better with external canister filters and occasionally with some internal filters with the right pipe size for their inlet.
Hope that helps.
Cheers,
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