Lack of surface agitation. There are a few ways people use to clean it (if it's really getting bad) :
- surface skimmer or extractor (usually use in conjunction with filter input tube.
- get mollies ( I heard the female ones do better job).
- adjust the canister output (this is what I do in my tanks) to give a little surface movement. A little surface movement wouldn't waste CO2.
A paper towel skimmed over the surface will work also. But in freshwater, its usually dust accumulated on the water and lack of surface agitation. If you wear lotion, that can also cause it if you put your hands into the tank.
This is caused by the rapid decay of waste, etc. and it does block light out somewhat. Nino's advice is really good. I tried the 'paper towel' method and it only cleared the skim for a couple of hours and then it was back. If you're going to get a surface skimmer ask around for tips on how to get it work(I heard it's hard to adjust).
This tank has been up for about two months now. The film layer appeared a week ago, before that, the surface was crystal clear. I have heard that too much light can be the cause of this. A month ago, I had 30 watts of normal flourescent, but recently I have added another 45 watts. A total of 75 watts over a 10 gallon.
I have a Fluval filter so I got me a Fluval service skimmer, works like a charm. No problem with adjusting once you get the skimmer set up like you want it. I have nerver heard of high light causing surface scum, only lack of surface agitation.
It is from Decay. Simply get your plants to grow better . Also t he fluval Surface skimmer is not hard to adjust its very simple. It works with Filstars as well. I do not think it will work with ehiem out of the box, some adjustments must be made. of course Eheim has their own.