Saltwater Fish

Lawnmower Blenny

Salarias fasciatus

Comical algae grazer with plenty of personality · Beginner

Lifespan: 4-6 years
Adult size: 10-13cm (4-5in)
Min. habitat: Aquarium 110L+ / 30gal+
Social needs: One per tank unless the system is large
Diet: Herbivore, marine algae
Time: Low
Cost: Medium

Overview

The Lawnmower Blenny is a hardy, characterful herbivore named for its habit of grazing algae off rock and glass.
With perching pectoral fins, expressive eyes, and constant activity, it is a popular beginner reef fish.

It is reef-safe and useful for nuisance algae, but it needs an established tank with a real algae supply.
Keepers must be ready to supplement food once the algae is grazed down.

Housing

House in a mature reef of at least 110L (30 gallons) with plenty of live rock covered in algae film and perches to rest on.
An established tank with diatom and green algae growth provides natural grazing.

Keep temperature 24-27C (75-81F), salinity 1.024-1.026, pH 8.1-8.4, and ammonia and nitrite at zero.
A tank that is too clean or too new will leave this grazer underfed.

Diet

Primarily a herbivore, it grazes hair algae, diatoms, and film algae from rock and glass.
Once natural algae is depleted, supplement with dried nori, seaweed sheets, spirulina flakes, and algae-based pellets.

Many lawnmower blennies slowly starve in tanks without enough algae, so a sunken belly signals the need to feed more vegetable matter.
Some will eventually accept meaty frozen foods as well.

Health

Hardy and relatively disease-resistant compared with many marine fish, but it still benefits from quarantine and stable water.
The most common cause of death is starvation in tanks with insufficient algae rather than illness.

Watch the belly profile: a pinched, concave stomach means it is not getting enough food.
Provide constant grazing material and supplemental nori to keep weight and energy up.

Temperament

Peaceful toward most tankmates but territorial toward other blennies and similar bottom-dwelling fish.
Keep only one lawnmower blenny per tank unless the system is large, to avoid squabbling.

It is an amusing, interactive fish that perches openly, hops between rocks, and shifts colour with mood.
It generally ignores corals and invertebrates, making it a reliable, peaceful reef community member.

A good fit for

Beginners wanting a hardy, fun fish
Reef tanks with nuisance algae
Keepers who will supplement with nori
Community reefs needing a peaceful grazer

Common mistakes to avoid

Starves in tanks lacking enough algae
Watch for a pinched, concave belly
Keeping two blennies together causes fights
Adding to a brand-new, algae-free tank

Is your enclosure big enough?

Minimum volume110 litres (29 US gallons)

CompanyMust be housed alone. One per tank unless the system is large

Check your own enclosure against this

Is this urgent?

In a reef, stability matters more than any single target number. A parameter moving fast does more damage than a parameter sitting slightly off.

Go now

Any detectable ammonia or nitriteWater change now with matched, mixed and aerated saltwater. Stop feeding. Directly toxic, and a marine system has less margin than a freshwater one.

Now

Temperature or salinity has moved sharplyCorrect slowly, over hours, not minutes. Check the heater and the top-off. The speed of the change does the damage. Correcting fast repeats the injury in the other direction.

Now

Rapid gilling, or a fish hanging in the flow breathing hardIncrease aeration and check temperature and salinity. Warm water holds less oxygen, and a heater failure shows up here first.

Now

Today

White spots, or a fine dusting like icing sugarIdentify which parasite before treating. Most reef tanks cannot be treated in place. The two common parasites look similar and need different handling; velvet can kill a tank in days.

Today

Corals closed for more than a day, or receding tissueTest alkalinity first, and compare it against the last reading rather than a target. Unstable alkalinity is the most commonly cited coral killer, and a swing shows up in the trend, never in a single point.

Today

A new fish added without quarantine now looks unwellRemove it to a separate system today if you can. Nearly every wipeout in a reef tank traces back to a skipped quarantine.

Today

Quarantine every new animal, including corals and inverts. It is the single practice that separates a reef that lasts from a reef that restarts.

This is general guidance for a healthy adult, not a diagnosis. An exotics vet who knows your animal always overrides it — and the time to find one is before you need one.

What it really costs

Setup, monthly, first year and across a whole life, itemised and with the electricity worked out from the actual wattage. Published figures for the same animal disagree by a factor of four, so the calculator shows ranges rather than picking one.

Work out the cost of a lawnmower blenny

See all Blennies side by side →