Eriocaulon quinquangulare
Also known as Eriocaulon sp. 'King Crimson' and Eriocaulon sp. 'Red India'. Among the few plants from the family Eriocaulaceae kept in planted aquaria, it is the only red one, making it demanded and expensive. Like most other Eriocaulon it a very peculiar plant: The Eriocaulon quinquangulare grows in (hyper-)oligotrophic lakes and wetlands submersed in a swallow depth or emersed, thus it has odd strategies of accquiring nutrients, especially Carbon. It is an extremely heavy rooting plants that can grow three times as much root mass as any other biomass and these roots all have a very high oxygen loss, so it oxygenates the soil and thus allows aerobe decomposition to release CO₂ which is also uptakes over the roots [1]. This nearly unique C uptake mechanism is limited to the so called Isoetids, a superficial groups at encompasses most Eriocaulon species and a few more. However, this comes with a hefty price: They cannot use HCO₃- for photosynthesis in the interestingly CAM-like metabolism. CAM is the water-saving metabolism of succulents and not what one would expect in an aquatic plant.
So what does that mean for the aquarist? It means to care for it an CO₂-System is almost necessary, but also that you can use them to aereate your substrate very effectively, but fertilization if not strict necessary when slow grow is tolerable. Many other aquarist also report that they cannot grow them well in water with a high KH/high concentration of HCO₃-. In my case it seems to do okay in 3° dKH water.
