More along the lines of testing a riverbed cap. I've seen journals over the years and I loved this look because it looks exactly like a faster pace river bottom while diving, but in an aquarium.
I've been capping all kinds of dirt since I was a kid. First PT journal though.
So far, I can say that I am not a fan of the river bed cap. It is always dirty. River bed substrate is so light in the water, almost like disturbing Laterite. Every time you mess with anything it makes a mess. However the consistency when pushing new roots into the substrate is like Traffic Bond. I heavily rinsed and sifted / discarded anything that would fit through a window screen.
I would strongly advise some kind of bleaching or H2O2 treatment or even boiling before using river bed gravel in a tank to kill off the organics, as this now decaying mulm, even 2 months later is still a PITA to siphon off the bottom, and is still showing up...
The Fraternity of Dirt #12
Six Quart Hex Journal