Hemianthus callitrichoides belongs to the family Scrophulariaceae and is native to Cuba. It is one of the smallest aquarium plants in the world. Its size makes it ideal for small aquariums, but it can also be used in larger, well-lit tanks. Its root system is only a few millimetres below the substrate. On the surface, it forms small, low creeping stems with tiny oval petals about a millimetre long. If it has suitable growing conditions, it thickens to a height of several (2-6) centimetres. It forms long rhizomes laterally, creeping along the bottom. Over time it forms a compact dense green ‘carpet’.
Its stature makes it suitable for the foreground of the aquarium. In the case of a small aquarium, it can also be planted across the entire width. Plant in small clumps 5-10 cm apart. I recommend using a pair of tweezers to make planting easier.
It thrives in stronger lighting, in poor lighting it will grow more in height and its growth will be slow. I’ve had a few clumps experimentally in a small aquarium just outside my window. No additional lighting, window facing north. As I planted it, I found it three months later – same size, faded, but alive. A hardy little plant.
CO2 is not a requirement (it produces beautiful oxygen bubbles on the leaves if CO2 is oversaturated). As for fertilizing, I personally fertilize very irregularly and it hasn’t had any effect on the growth rate – it grows great. Of course the situation will be different in every aquarium, keep an eye on the growth and colour of the leaves and adjust the conditions accordingly.
Be careful when selecting fish – the roots are very delicate and it is easy for the plant to detach from the substrate. Choose a fine substrate, with a maximum grain size of a few millimetres. If the substrate is coarser, you can help the plants by pressing them down with a small pebble, a piece of glass, etc. You can also attach them with thin fishing line for decoration – root, stone, etc.
If the plant grows rapidly, whole sections of the previously compact ‘carpet’ may start to loosen from the substrate. The plant becomes so dense in height that its lower parts do not get enough light and its roots begin to rot away. (Something similar happens to riccia.) Hemianthus therefore need to be occasionally broken up and thus over-watered.
If you are just starting out in aquarium keeping, I recommend waiting to buy this species until the aquarium is established. In case of problems with the algae, use caution. It is a very tiny and fragile plant, mechanical cleaning from green algae is almost impossible. To help with cleaning I can recommend from the SAE fish (Crossocheilus siamensis, unsuitable for small aquariums), from invertebrates snails reel snails, Physa and a great green algae harvester – the shrimp Caridina japonica. However, in the case of unsuitable conditions in the aquarium, where we do not deal with the cause but the effect, they will not help us much. Just don’t panic, don’t pour more fertilizer/CO2 if Hemianthus callitrichoides is not growing to your liking. Its growth is very slow for the first month – transitioning from an emerse form to a submerged form. Then a breakthrough occurs and Hemianthus callitrichoides floods the aquarium (if conditions are right for it, of course).
If you can’t grow it in an aquarium, try it emerged. Small aquarium without water/pot etc, soil (experiment when choosing, some use soil for palms, others for cacti etc). Cheap artificial light of daylight colour, alternatively put the container in a well lit place (window, balcony, garden). If it thrives, it will bloom with tiny white flowers.