Lawnmower Blenny
Salarias fasciatus
A goofy, perching algae-grazer that mows film and hair algae off the rocks.
Difficulty: BeginnerReef-safe community fishReef-safe
The lawnmower blenny (Salarias fasciatus) is a goofy, big-eyed algae-grazer that perches on the rocks and “mows” film and hair algae all day. It's peaceful and full of character — but it's not a magic cleanup fish: it needs a mature tank with real algae (or supplemental nori) or it will slowly starve.
Care at a glance
| Adult size | Fairly large for a blenny — about 4–5 in (10–13 cm). |
|---|---|
| Lifespan | About 2–4 years (some live longer). |
| Minimum tank | A mature 30–55 gallon tank with plenty of live rock and algae to graze (55 gal is the common official recommendation). |
| Temperature | 75–82°F (24–28°C). |
| pH | 8.1–8.4. |
| Specific gravity | 1.020–1.026 (≈35 ppt; keep it stable). |
| Other targets | Needs an established tank (roughly a year old) with a real microalgae film to graze; keep nitrate low. |
| Diet | Herbivore/algae-grazer — filamentous algae, diatoms and detritus off the rocks. It will take dried algae (nori), spirulina and some frozen foods, but it MUST have grazing algae or supplements or it wastes away. |
| Temperament | Peaceful with other fish — but territorial toward other blennies and algae-eaters that share its niche. |
| Reef-safe | Yes — it grazes algae, not corals. (A starving one may occasionally pick at polyps — the fix is to feed it.) |
| Natural range | Indo-Pacific reefs. |
The fish store gets these wrong
Common myths, kindly corrected — no shame for having believed them.
- “It's a cleanup fish — it'll live off algae.” Only if there's enough. In a new or spotless tank a lawnmower blenny starves. It needs an established algae supply plus supplemental nori.
- “It'll eat any algae problem.” It grazes film and some hair algae, but it won't clear bubble algae or a bad outbreak — it's a helper, not a cure.
Before you buy
- Only add it to a mature tank with real algae growth.
- Have nori/algae sheets and spirulina ready to supplement.
- One blenny per tank (they scrap with other blennies/algae-eaters).
- Don't buy it to fix a big algae outbreak — it's a grazer, not a cure.
Quarantine and get help if…
We can't diagnose from a photo — but these are the signs to stop watching, quarantine, and get an aquatic vet or an experienced reefer's eyes on it:
- A pinched-in or hollow belly, or a visibly thinning body — the classic sign it isn't getting enough algae; start supplementing now.
- White salt-grain spots, flashing, or rapid gill-breathing (possible ich or velvet).
- Cloudy skin patches, clamped fins, or gasping at the surface.
- Any new fish added without quarantine.
Never
- Never add it to a new or algae-poor tank expecting it to survive on nothing.
- Never rely on it to clear a serious algae outbreak.
- Never keep two blennies of similar type in one tank.
- Never add it to an uncycled tank.
Common lawnmower blenny questions
Do lawnmower blennies need an established tank?
Yes — this is the big one. A lawnmower blenny grazes film and hair algae, so in a new or spotless tank it slowly starves. Add it only to a mature tank with real algae growth, and supplement with nori/algae sheets.
Are lawnmower blennies reef-safe?
Yes — they eat algae, not corals. A starving one may occasionally pick at coral polyps, so the fix is simply to keep it well-fed.
What size tank does a lawnmower blenny need?
A mature 30–55 gallon tank with plenty of live rock and grazing algae (55 gallons is the common official recommendation). It needs surface area to mow.
Will a lawnmower blenny fix my algae problem?
It helps with film and some hair algae, but it won't clear bubble algae or a serious outbreak — treat it as a grazing helper, not a cure.
How long do lawnmower blennies live?
About 2–4 years, sometimes longer with a steady food supply.
Sources & further reading
This guide was written and fact-checked against these authorities. It's honest, general guidance — not a diagnosis, and not a substitute for an aquatic vet or your own test kit:
- Saltwater Aquarium Blog — Lawnmower Blenny Care — size, lifespan, tank, algae diet & starvation risk, temperament, reef-safe, range
- Tropical Fish Hobbyist — Uber Algae-Eaters: The Lawnmower Blennies — algae diet, starvation risk, husbandry
Written and fact-checked by The Coralkin Team against marine-aquarist and veterinary sources — if your lawnmower blenny shows a red-flag sign above, quarantine and get an aquatic vet or an experienced reefer's eyes on it. We never guess a care number.
🔬 Review status: team-written and fact-checked against cited sources · not yet reviewed by a marine vet. How we research & fact-check →