I guess its probably not the chemicals themselves which cause the oil but rather their reaction to certain compounds in the water which result in oily by-products... same for bacteria solutions, they work to further breakdown the organic waste hence more oil generated. Basically as long as there are living things and food in the tank, a portion of the oil they produce will float up to the surface. Humans also have to combat oily skin so its a constant annoyance in life too.
Overdosing the chemicals may not cause as much negative effects (unless massively overdosed), but it could increase TDS quite a fair bit in the change water, so perhaps better to follow the recommended dosage to keep things moderated.
Yeah, the Boyu EF-05 is a really low flow rate canister filter, i used it before and its 150 l/ph flow rate was slow enough (and after packing with media became even slower) that i could even use it in a betta tank at its max flow (though i'd say those who keep betta tanks can definitely consider this filter).
Maybe instead of getting a powerhead to increase its flow, why not just upgrade to a larger canister filter with higher flow rate, then you'll benefit from both higher flow rate + larger bio-media volume.
Yes, those small internal filters with sponge containers can be used as surface skimmers too... some people do that and i've personally tried it out too, just mount it upside down with the motor and outflow at the bottom, then position it so that the sponge intake grills at the top is at the surface water line, then it works exactly like a surface skimmer. Not as fancy as other surface skimmers like the Eheim 350 or ADA Vuppa, but it'll work.
Just remember to wash and clean the sponges regularly, as they will clog up quite fast with debris and oil. In addition, do put some fine sponges into the intake container so that shrimplets or small fishes don't inadvertently get sucked into it.

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