Udotea sp.
| Care Level | Moderate |
| Lighting | Moderate to high |
| Water Flow | Moderate |
| Placement | Sand bed (roots via holdfast) |
| Primary Use | Ornamental display |
| Growth Rate | Slow |
Overview
Mermaid’s Fan (Udotea), also known as fan weed or mermaid’s tail, is a striking calcified green macroalgae shaped like a delicate fan or paddle on a short stalk, anchored into the sand by a root-like holdfast. Like Halimeda it lays down calcium carbonate, giving it a firm, slightly crusty texture, and it makes an elegant, sculptural display plant rather than a fast-growing nutrient scrubber.
Care & Placement
Moderate. Give it moderate to high light and moderate flow, and plant the holdfast into a sand bed — it roots into substrate and will not thrive glued to bare rock. Because it calcifies, it draws on calcium and alkalinity as it grows, so keep those stable just as you would for corals and Halimeda. It is slower and more ornamental than Caulerpa.
Uses & Nutrient Export
Its main value is ornamental — a natural, sculptural look for a display or refugium. It offers modest nutrient uptake but is not a heavy exporter like chaetomorpha or caulerpa. Being calcified, most herbivores leave it alone, much like Halimeda, so it survives in tanks with grazers.
Propagation
Slow. It spreads by sending up new fan blades from runners in the sand, and can be propagated by dividing and replanting holdfast sections into the substrate. Give new plantings time to root and establish.
Cautions
It needs a sand bed to anchor in and stable calcium and alkalinity because it is a calcifier — it will languish on bare rock or in swinging chemistry. It is slow to recover if damaged. On the plus side, unlike Caulerpa it does not “go sexual,” so there is no risk of a sudden crash.
Frequently Asked Questions
Is Mermaid’s Fan hard to keep?
It is moderate. Its needs are a sand bed to root in, moderate-to-high light, moderate flow, and stable calcium and alkalinity (it calcifies like Halimeda). Given those, it is undemanding — just slow-growing.
Does Mermaid’s Fan need a sand bed?
Yes. It anchors into the substrate with a root-like holdfast and will not thrive attached to bare rock. Plant the base gently into a sand bed and let it establish.
Is Udotea good for nutrient export?
Only modestly. It is a slow-growing, calcified ornamental alga, so it is grown for its sculptural look rather than heavy nutrient export — choose chaetomorpha or caulerpa when export is the goal.