So its a hang on the back filter then....so does the water return disturb the surface? What about an air stone?? You need something that will surface disturbances. You can also invert the gravel vacuum and suck it up. Another option is and I have done this several times. Take a paper towel fold it up soo it fits the width of the tank. Then "wipe" the surface of the water. It will get the scum off. Its kinda tricky at first but it is effective at removing it.
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