Floating plants (like Duckweed) getting caught in the filter outflow causes them to get flung into the water, where they usually get sucked up by the filter. This solves that by keeping them away from the waterfall.

Additionally, it floats on the surface so there shouldn't be any problems due to filling, evaporation, or water changes. The floating range is about 120mm, or 5 inches.

I saw AquaClear-50 Floater Barrier 180cm by henryloc on Thingiverse and tried to print it, but it was too large for my Prusa MINI bed. I redesigned it myself to be sized so that one side fits under the filter itself (between the intake tube and output flow).