These are the two carpet plants everyone asks about, and the honest answer is that they are not interchangeable. One is forgiving. One is not.
Monte Carlo — the one we recommend
Micranthemum tweediei grows horizontally under medium light, tolerates a wide temperature range, and recovers from mistakes. If you forget a water change, it sults for a week and then carries on. It is the right choice for a first carpet, and for anyone who does not want to run CO₂ at 30 ppm with a pH controller.
HC Cuba — the one that punishes you
Hemianthus callitrichoides has smaller leaves and a tighter, more refined carpet. It also melts in low light, dissolves without CO₂, and turns into a floating mat if the flow is wrong. Under a strong LED with reliable CO₂ it is stunning. Under anything less it is a slow, expensive death.
A practical test
Be honest about three things:
Light. If your bar gives less than 60 µmol at substrate, choose Monte Carlo.
CO₂. No injection? Monte Carlo.
Consistency. If you travel, or if this is your first planted tank, Monte Carlo.
Getting either one to carpet
Plant in dense portions rather than individual stems — six to eight plugs per cup, not sixty. Keep the light low for the first two weeks while roots establish, then raise it. Trim at three weeks even if it looks short: cutting forces the horizontal runners that make a carpet. And be patient for the ugly phase, which lasts about a fortnight and ends suddenly.