[PhaneSoul]

I would just try to make it bigger. I took a pump off a hob filter, inverted it and added a box to it to surface skim, works great at the right water level, def think size or a valve is the key to what you (we) are doing here. Mine iz the same exact con ept but im just circulating the scum on top and you run it into a filter (i also have the option of attaching a filter sock to mine)

 

[PhaneSoul]

The part that actually draws in water from the surface. If you can get that part to retain water & not go dry while being turbulant i think it would work better. Maybe try a square instead of a circle, a cirlce seems like it would help create the vortex, my square doesnt create a vortex but due to tge turbid water in it and shallowness if the waterlevel drops too low it sucks air (no sump so water level in display fluctuates)

 

[PhaneSoul]

Here is mine

If i rebuild it (which i will one day) ill design it so it has an inner box as well, the inner box will float with the water level while surface skimming