Is it actually oil?(rainbow) or is it just protein build up?(more white when enough builds up)

When protein film is broken via touching it or surface agitation it will clump up into white flakes.

If it is protein, then there's just an excess of either food or decomposing plant matter. I've been dealing with it and it can be dealt with by either using a surface skimmer/hob filter to increase surface agitation or by laying a paper towel on the surface after water changes. At least until the source is either cleared up or discovered. Having floating plants may make dealing with it difficult though.

 

or by laying a paper towel on the surface

Bonus for this trick: gently squeeze the paper towel out and let it dry off. Can be reused to get surface film for quite a while. Doesn't work all the time, of course, but it usually does. Even with store/generic varieties.

Thought I'd share in case anyone else enjoys trying to avoid waste.

 

Yeah, I get a surface film on my established tank from time-to-time, too, and from what I can tell it comes leftover from fish food, excessive organics in the water column, decaying leaves, etc. Solutions include more frequent water changes and filter cleaning, increased surface agitation (for better O2), purigen, and surface skimmers (I have an eheim)

A search here on the forums turned up a lot of hits on this topic

 

Also some oils from your hands if you stick 'em in the water. I get rid of it the slow way....filling up a shot glass which is levellled against the water surface time and time again. Slow, but works!

 

Also some oils from your hands if you stick 'em in the water.

I have never believed this. You'd have to be Pizza the Hut to give off enough oil to make a showing at the surface. Human beings just aren't that oily. Nope. Not for me. Myth zone. (lest anyone waste time or money wearing gloves when doing "tank stuff")

That being said, I recently dealt with this. Every morning, I got a thick white film. It only showed up after the lights and CO2 went off. For me, it was my rich substrate. I have a substrate that is a mix of earthworm castings and black fluorite sand.

My solution was just to get a fish net - the kind you dip floaters or plant trimmings with - and used it to skim. It went on for about 10 days, and then no more.

 

I looked closer and it's actually white- I suppose it's the decaying plant matter, I have a pretty full tank with plants and some are always decaying

 

surface scum is common in planted tanks, and can very very dangerous if you're injecting CO2. I think most people use surface skimmers, either standalone skimmers like the Eheim Skim 350 or lily pipes with a built in surface skimmer.