HC cuba is baby tears, they find it hard to grow without co2 though. a better choice would be glossostigma elatinoides they look like HC cuba but glosso grows better without co2 , hair grass i found grows badly without co2 better choice again would be sagittaria subulata or Echinodorus tenellus
Staurogyne repens will also grow as a "carpet" but it is a stem plant and will require trimming back/ replanting to be kept compact
i feel the easiest carpet is riccia, or a moss of some type