All the mucus and fats that are produced by the livestock will float on the surface-- the old "oil and water" trick. That's the idea behind the surface skimming overflow. Surface agitation can help but it really just pushes it around if there's nothing to literally pull water from the surface to the sump or filter.
If you dont have an overflow you can get a surface skimmer thats basically just a powerhead that sucks water off the surface and pumps it back into the tank
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