Thick Brownish Oil on surface of water

[Phillipians]

I recently noticed that my tank water is full of surface oil (or something like oil). It stains the glass as it evaporates. It is easy to wipe off. However I thought my ANS brand steel surface skimmer would get rid of it. However it is not really doing a good job... Regret buying it a little as it was pretty expensive.

1. What exactly is this layer of oil?
2. Any way to get rid of it easily?

My fish look ok. No gasping. All look healthy. In fact ironically, this tank is the best I have done up for a high tech tank - *fingers crossed* no algae yet except of course the usual GSA. The plants are all super healthy.

I was wondering if the ADA soil has anything to do with the oil as the layer of film looks very similar to the layer after I started the tank with ADA. It has been 3 - 4weeks now. The water has cleared. Not sure why the brown film is there.

Substrate: ADA amazonia
Filter: Eheim 2217 (Matrix and Substrat pro)
Lights: Maxspect 120W 8k. On R70% B80% for 8 hrs a day.
Plants: APP, HM, Narrow Leaf, Moss. Another stem plant that I have not identified yet.
CO2: 3BPS
Tank 2x1.5x1.5ft
Ferts: Lushgrow Macro and micros plus some ferts. Also added in 5 drops of EHIO Iron every water change.
Water change 1-2 times 40% each time.

[Urban Aquaria]

If the oil layer is visibly brownish, it could be the oils from the substrate still being released, then its just a matter of removing them and eventually they will not appear anymore.

If its more like those translucent surface oil film, then its quite normal, especially if you have a planted tank with regular fert dosing. I guess there is a good reason why ADA also came up with their Vuppa-I surface skimmer too.

More water movement on the surface helps to dissipate the normal surface oil film... though i've found the best method to minimize it is to do regular water changes, i've observed that if i skipped a weekly water change in any of my tanks, i'll start to see a visible oil film start to build up in that particular tank's water surface. If the oil layer still persists, then have to increase the water change frequency.

Surface skimmers do help too, but if the oil layer gets too thick sometimes even the skimmer is abit overwhelmed.

Btw, i've tested having de-chlorinated water sitting in a plastic tub (was testing if it was good enough to use as a water garden), it had no plants, no fishes, no soil and no filter, just a tub of standing water... after a few days, before i poured away the water (to avoid breeding mosquitoes), i checked and i could also see a visible oil film on the water surface too.

[Phillipians]

I see. Strange that it started giving out the oils again then. It wasn't doing that the past 1 week and suddenly it was there again...