Although many factors can cause the oil slick, the main source is from the fish food. Almost all kinds of fish foods contain fat in their ingredients, fat provides fish with an easily accessible and highly nutritious source of energy for movement and metabolism. Fat from fish waste and food-remains will float & form an oil slick. The oil slick gets thicker everyday, surely an eyesore to your planted aquarium.

Surface scum is bubbles on the water's surface that won't easily break, it will accumulate along the surface. In severe cases, all surface will be covered with scum! Some factors can cause this problem, but if you had an Oil slick problem, surface scum will surely come along with it. The bubbles won't break because they are covered by slick - could be oil, dust or others, it will stay and accumulate.

Still water (calm and not flowing water) will surely form a slick that covers the entire surface. There are 2 ways to prevent the slick formation:

Break the surface by using an aeration, splash current or venturi-output. Surface agitation only won't be effective to break the water's surface, especially if you apply a gentle-agitation. Vigorous surface agitation should be done together with an aeration / splashing water/ venturi output. The vigorous agitation purposes are for the gas exchange and to stir up/ mix the surface with the rest of the water, then it will be filtered.
Continuously slurp the water's surface to be skimmed or filtered.

You might already heard some prohibitions in aquascape such as: " Put the water outflow of any filters about 2 cm below the water's surface so that it does not produce a splashing current on the surface. This enables sufficient oxygen to be absorbed by the water without expelling CO2. Do not use aerators, there is always sufficient oxygen available for all inhabitants in a well-planted aquarium with CO2 nutrient."

Are the above prohibitions 100% true?

Continuing O2 & CO2 roles article, the remaining oxygen during the night might not be sufficient for all fishes and other organisms. If reducing the inhabitants is not an option, you should add dissolved oxygen into your tank - it also means throwing away the CO2 that needed for the next-day photosynthesis.

But, even if you decide to reduce the inhabitants or extremely keep the freshwater aquarium plants only, you still can't escape from the classical problem: slick formation!

So, what are the solutions?

To settle the contradictory implication between keeping the CO2 and supplying oxygen within the planted aquarium, you should use a day/night timer to operate the devices by turns. An Aquarium CO2 system will be set for 10-12 hours running during the day (for photosynthesis) and agitation/oxygen-generated devices (such as wavemakers, powerheads or aerators) will take turns at night for respiration. Of course, during the night, the CO2 from freshwater aquarium plants and other organisms' respirations will be thrown away. Aeration and agitation will do their functions and keep the water's surface free from any slicks and scum.

I should remind you that the CO2 levels strongly affect the aquarium pH.

You should regularly check your aquarium pH to keep it stable (ideal pH range : 6.5 - 7.5). But a day/night timer can't monitor and adjust the amount of CO2 being released!. If you're not on a tight budget, use an Automatic aquarium CO2 system that can automatically adjust the CO2 output to get the desired pH.

If the pH value jumps above 7.5 due to aeration/ surface agitation during the night, you should consider to take the automatic aquarium CO2 system or the 2nd solution.

If the normal oxygen level [about 5-8 mg/liter (ppm)] can always be maintained, you don't need any additional oxygen-generated devices. But the slick formation on the water's surface keeps on going and the slick gets thicker everyday. For surface problems, we may use devices which can suck all the surface materials (such as dust, dirt, dead small-insects, etc. including all slicks s.a oil or protein slicks) and eliminate them.

Aquarium protein skimmer is one of several devices which can prevent water's surface problems by sucking the water (including all materials) on the surface and turning it into the skimmed-water. The aquarium skimmer works by aerating the water in a column to create a bubble rich froth which attracts waste/surface materials and then is pushed into the collection chamber. The waste-froth will cling to the chamber's wall and dry there. That's how the device skim the water's surface. This device also adds some oxygen (but not as much as a direct aeration) into your tank since it uses the aeration work-system.