Join Date: May 2020
Location: Southeastern US
I also have a 20 gallon long and my HOB provides enough flow to keep the surface clear. However, I have floating plants that the filter tends to push underwater, so I have been experimenting with different ways to prevent that from happening. Often times these measures restrict the water flow in one way or another and that's when I started the see scum. My tank isn't any dirtier on the whole, but I'm sure that organics are accumulating in certain places now that they weren't able to before. I look at this as more a flow problem than a cleanliness problem, but I suppose that's a matter of perspective.
I haven't figured out exactly how to handle this completely in my own tank (I suspect I'm either going to have to remove the floaters or just accept a certain amount of scum), but I have found that if I only feed in areas with good flow it really slows the rate of scum build up. Also, if you float a paper towel on the scum it does a pretty good job of clearing the area quickly, but that's only a temporary measure.