Layer of oil on my tank?

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Aquarium Advice Regular
Mar 19, 2023
This morning I noticed a slight film over my betta's tank. By looking it up, It said that it was from either an oil or protein build up. I haven't had this happen before.

This is a 10g tank holding 1 betta. I last changed the water 2wks ago ( i usually change evry 3wks)

I feed him fluval bug bites and North fin betta bits - I'm not sure if those are oily and could be the reason. I also don't have any nitrate issue ( literally stays at zero and have to add nitrates to feed my plants) so i'm not sure if it would be from oily poop. I use Easy Green fertilizer.

I heard you could absorb it with a paper towel, so that's what I did. Here's a photo of the section I didn't get to.

When i stir it with my finger, it clumps together.
I also read that it could be due to water flow, which would make sense because I just added a sponge to help slow it down (you can check my other threads). But my problem is that I would need to keep the sponge in because the slowness of the filter keeps my betta comfortable, so I'm more so looking for whats causing it and wanted to know how to cut it off from the source.

Is it dangerous to the fish? and how do I stop it?
Its protein build up, probably from food. I have a granular food that causes that film if i break up the granual. Good surface agitation from the filter breaks it up and prevents it from forming. You can remove it with a paper towel as you mention. I find doing water changes by scooping out water with a jug rather than syphoning out the water removes it, but it will come back. Ive never had it cause a problem for fish, but if it built up too much it could slow down oxygen transfer into the water. Try different food, or periodically remove it.

Its strong flow that bettas dont like, you can still get good surface agitation while slowing down or dissipating the flow. Try lowering the water level a little so the water output from the filter ripples the surface more.
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