Chain sword or Echinodorus Tenellus grows like stink. Probably the fastest growing carpet plant you can find. If you don't find someone selling carpet I say it's worth investing in 3 pots of the stuff. They grow well in low light and with CO2 they'll invade every corner of your tank in relatively little time.
Glosso grows tall in low light. I wouldn't recommend it.
Otherwise, marsilea minuta is another good option.