this is interesting....i have a planted 10g betta tank with play sand, and my dwarf sags and narrow leaf chain swords are really growing like crazy. i was intending on adding a root tab....but haven't at this point. i use liquid ferts occasionally. i wonder if i added a root tab (seachem), if i would see even crazier growth.
but for being such an inert substrate, those plants are doing rather well....and have been for about 6 months. they were slow to get going...especially the chain sword, but now, both are spreading all over. which is good, because i'll take some of them to start my 10g NPT.