It was a bit strange situation, I had no problem at all for almost a year with the cleanliness of the water surface in my aquarium. I did not do any changes to my tank and one day I realised some bubbles starting forming, after a week the situation was a bit serious, especially my fish started to stay close to the water surface – a typical sign of the oxygen lack…
A quick research did not help. usually people complain about scum made from a biological, protein film, but the film, what I found on internet, looks more like this:
Well, I still do not know what caused the bubbles in my aquarium. All water parameters were absolutely fine… But I bought a small Eheim Skim 350, which is a small surface skimmer, and it resolved the problem (but probably not a cause!). It took about 5 days to make the water surface clean. Also I decided to replace the original sponge with much dense sponge, to make sure the small particulars will stay inside. Also for the first 5 days I kept cleaning the sponge on a daily basis. Anyway, surface looks fine, fish are happy 🙂
Small edit: Eheim Skim 350 is not large, but in my small fish tank it looks quite large. So what I decided to do was to but a very cheap surface skimmer from eBay – see the picture below. I connected the tees fitting to the pipe sucking water into the external filter. Unfortunately I could not buy the surface skimmer anywhere in the UK so I had to wait few weeks for the parcel from China. But the result is great. Instead of the large pump with skimmer I ended up with a small additional pipe attached to the filter pipe.
The only disadvantage was that the external filter pipe is diameter 16mm – exactly the same as the bottom part of the tees fitting, bit the upper part of the fitting is 18mm internal diameter so I had to use a little bit of the electrical insulation tape plus a bit of super glue and on top of everything I used (I am not sure why) the heat shrink. I hope I gave you some good ideas about the surface skimmer.