I experimented with sustainable food sources for my shrimp aquarium which I usually don't feed since the population grows slowly with the natural algue supply, so I put in some potato peels which apparently contained a lot of oil on them which now floats on the surface for some days. How can I remove it without a special device like a skimmer?
Get a (clean) bottle and push it straight down, with the opening facing upwards, so that the opening is barely under water.
It will slowly fill the bottle with only water from the surface, including the oil.
It's important that you don't push it too deep. There's no need to move the bottle around, just keep it in one place, and everything will be sucked into it.
Another idea is to use a paper towel and dip it in the oil. but again you have to do it gently so it doesn't take too much water instead of the oil.
Cleaning the surface with a vacuum turned out to be extremely annoying since the tube needs to be hold in the right position in order to avoid too much suction of air which requires to restart the vacuum every few seconds with few oil removed.
Laying paper of the surface turned out to be a good idea because it sucks in the oil before the water below. Remove the paper fast and let it drip off outside the aquarium in order to avoid as much oil getting back as possible.
This should work for any surface (protein, bacteria, etc.) films.