There is barely any surface movement, but not much scum right now. As protein film accumulates surface tension increases, allowing little snails to crawl along the top and eat the scum.
In the outdoor mud bucket, the platy fish eat scum more efficiently and it doesn't have a chance to accumulate, the fish swimming / zipping back and forth also creates sufficient "flow" for ammonium to distribute to the plants.
I'm in Orlando and also have outdoor planters with no filtration or lights. The only inhabitants are snails and some RCS culls from my tank in Chicago and I also have no film on the surface of the water. The water is crystal clear with only top offs from the rain.