Sea Urchins: The Best Algae Eaters You'll Love to Hate
Urchins are the most effective algae eaters in the marine hobby. Nothing removes algae as thoroughly as a hungry urchin. They're also the most chaotic invertebrate going — picking up frags, knocking over coral, and rearranging the aquascape like tiny, spiny bulldozers.
The Good
Algae annihilation. A single tuxedo urchin will clear hair algae, film algae, and coralline from rocks with mechanical efficiency.
Completely reef-safe in terms of coral predation. They graze algae off rock surfaces, not coral tissue.
Fascinating to watch. They pick up shells, frag plugs, and rubble to carry on their spines — camouflage behaviour from the wild.
The Bad
They eat coralline algae. That beautiful purple and pink coralline? Dinner. No distinction between nuisance algae and desirable coralline.
They rearrange everything. Anything not glued down gets moved.
Spines damage coral. Physical passage across rockwork can scrape SPS tissue and irritate LPS.
Species Guide
Tuxedo Urchin — The go-to reef tank urchin. Small (5–6cm), colourful, and less destructive than larger species.
Pincushion Urchin — A bit larger with short spines. Good grazers but more collateral damage due to size.
Long-Spined Urchin — Effective but those spines are sharp enough to puncture skin. Best avoided in tanks that need frequent hands-on maintenance.
Pencil Urchin — Thick, blunt spines. Less efficient grazers. Generally not the best choice for mixed reefs.
Making Urchins Work
- Glue down frags — anything unsecured will get relocated
- Accept some coralline loss — new coralline grows back behind the urchin
- One per 150–200 litres — avoid overstocking
- Supplement with nori if algae runs low
- Provide hiding spots — urchins are nocturnal
If a tank has minimal algae, an urchin will just eat coralline and bulldoze the aquascape. They're best deployed as a solution to a specific algae problem, not as a permanent CUC staple.
Anyone running urchins in their reef? Worth the coralline trade-off or more trouble than it's worth?
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