Sagittaria subulata could sometimes be troubling in terms of it's height. You could try Marsilea hirsuta but daily use of excel would be mandatory for any carpeting plant. If you like carpeting plants why not consider CO2?
When you say carpet do you envision small leaves like grass, or tiny stems like baby tears? If you just want green cover and dont mind big leaves, I grow wisteria sideways in my sand. Its covered a third of my tank in less than 3 months. I just evicted a bit of it.