HC Cuba: the ultimate aquarium carpet plant
At 2 mm per leaf, it is the smallest aquarium plant widely available — and the most demanding. When the conditions are right, nothing else comes close.
Where it comes from
Hemianthus callitrichoides — universally abbreviated to HC or HC Cuba in the trade — was discovered in the early 2000s in Las Pozas, a spring-fed pool in Pinar del Río province, western Cuba. It has since been found at a small number of other sites in Cuba, always in clear, warm, shallow water with high light reaching the bottom.
Its natural habitat gives you the care requirements in summary: the water in Las Pozas is warm, clean, and well-oxygenated, the light is intense (direct tropical sun reaching shallow water), and the substrate is fine sand or fine gravel. Replicating those conditions in an aquarium is the whole challenge — and the whole reward — of keeping this plant.
What to expect
HC Cuba is a creeping stem plant with round, paired leaves roughly 2 mm across, arranged opposite each other on thin, trailing stems. Under ideal conditions the stems spread laterally across the substrate, rooting at nodes and building a dense mat that lifts off the bottom by 2–5 cm. The overall effect is a bright, fine-textured lawn unlike any other plant in the hobby.
Without adequate light and CO2, the stems grow upward rather than laterally — reaching for light rather than spreading. The result is a wispy, upright clump rather than a carpet, and the plant tends to rot at the base once it blocks its own light. This upward-only growth is the most common sign that conditions are inadequate.
There are no established varieties. Hemianthus micranthemoides (pearl weed) is a related species sold as an alternative — it is larger, faster, and more tolerant of moderate conditions, but does not form as fine or dense a carpet.
How to keep it
| Parameter | Value |
|---|---|
| Lighting | High — minimum 50 PAR at substrate level; grows upward rather than spreading in lower light |
| CO2 | Required for reliable carpet growth — 20–30 ppm; without it, expect upright growth only |
| Temperature | 22–28°C |
| pH | 6.0–7.5 |
| Hardness | Soft to moderately hard (2–15 °dH) |
| Fertiliser | Regular micro and macro dosing needed; fine-grained active substrate preferred |
| Substrate | Fine aquasoil or fine sand — coarse substrate traps debris and prevents runners from rooting |
| Growth rate | Moderate when spreading laterally; fast upward when light is insufficient |
| Placement | Foreground |
| Difficulty | Advanced |
The light-at-substrate rule
HC Cuba needs high light measured at the substrate, not at the water surface. In a 50 cm deep tank, even a powerful light may deliver insufficient PAR at the bottom. The practical solution is either a shallower tank (30–40 cm), a high-output fixture positioned close to the water surface, or both. If HC Cuba is growing upward, the first thing to check is light intensity at substrate level.
How to plant and grow a carpet
HC Cuba is almost always sold as tissue culture (TC) — small, clean, gel-free portions in cups. Rinse off the culture medium, separate the mass into small portions (thumbnail-sized clusters of around 10–15 stems), and plant them approximately 2 cm apart across the foreground area. The small plant portions need to be anchored; use aquascaping tweezers to push stems a centimetre into the substrate.
The first 2–3 weeks are the critical window. Plants may melt slightly — this is normal as tissue-culture grown plants transition to submersed conditions — but as long as some stems remain green, new growth will emerge. Once runners begin spreading laterally across the substrate surface, the plant has established and will carpet reliably.
A weekly trim once established — cutting the carpet down to 1–2 cm height — keeps it dense. Without regular trimming the mat becomes thick enough to trap debris, decay from the bottom, and lift off the substrate in patches.
What goes wrong
Growing upward, not spreading. Insufficient light at substrate level, or insufficient CO2. Address whichever is limiting — usually light. See the highlight box above.
Melting after purchase. Normal for tissue-culture plants transitioning to submersed growth. Keep CO2 stable, maintain good light, and wait. Do not replant or disturb during this phase.
Brown patches lifting off the substrate. Debris trapped under an overgrown carpet causes the base to rot. Trim regularly and use a turkey baster or thin siphon to remove debris from between stems during water changes.
Stunted or pale growth. Usually a nutrient deficiency — most commonly iron or potassium in active substrates that have been running for over a year. Add root tabs and increase water-column fertilisation.
Snails and algae. The fine leaves trap algae and detritus easily, and small snails (bladder snails, trumpet snails) can damage the carpet. Nerite snails or careful manual removal are the practical solutions; avoid large or destructive snail species.
More plants in this series
- Staurogyne repens — the forgiving foreground plant
- Monte Carlo (Micranthemum tweediei) — fine-leaved carpet, moderately demanding
- Pogostemon helferi (Downoi) — the star-shaped foreground plant
- Eleocharis parvula (Dwarf hairgrass) — the grass-effect carpet
- Java moss (Taxiphyllum barbieri) — the moss that goes anywhere