I have a discus tank with pool filter sand and my sag. subulata grows great,root tabs can't hurt although I only use them for the swords just don't overdo and some water column ferts should be used also...
Eco-complete is pretty much inert so it would be the same as planting it in sand nutrient wise. Another factor probably affected the growth of the Dwarf Sagittaria.Mine grew taller in eco-complete, and spread quicker and stayed dwarf in sand.