[Kathyy]

Sand hasn't good CEC. Flourite and Eco-Complete do, they are clay or lava based.

You could put laterite [iron rich clay] and/or peat and/or mulm from an established tank under the sand as well as the root tabs. Not much, just a scattering of whichever you have or decide to use. That would increase the CEC of the substrate and mulm would mature the tank faster too.

 

[Kathyy]

No black substrate here but I have had carpets for a while and hairgrass has rooted in anything from pea gravel to raw rice grain sized Turface type stuff, never had sand though. It looks better with small grained substrate as those leaves are so fine. My substrate hasn't been rinsed out since I set up the tank about 12 years ago so loads of mulm. There was laterite in there at some point too. Occasionally I stick root tabs in as well. Tank has been heavily fertilized and CO2 injected for over a decade.

For staying down it is probably six of one and half a dozen of the other. Coarser material won't move as much but tiny hairgrass crowns might float up when you or a fish brush up against them as not as many roots are anchored. Fine stuff would hold the tiny roots great but it moves around easily so hairgrass roots might get dislodged. High CEC substrates are porous. Plants in the tank will grab bits of porous substrate more than the gravel. Since my substrate is a mix of gravel and high CEC stuff it is noticeable when I pull up a mat of hairgrass, more SAS than gravel on the roots.

Compare volume to price, sand is heavier per volume unit than Flourite or Eco Complete. The designed for a planted tank substrate may not be as expensive as you think it will be. Here is the calculator on this site. http://www.plantedtank.net/substratecalculator.html