Lawnmower Blenny

The lawnmower blenny is the fish people buy to clean their glass. It is very good at that, and it is also the fish most often bought for the wrong tank. Gallery Aquatica stocks them at our shop in Tingalpa, east of the Brisbane CBD.

Marine fish are in-store purchase and pickup only. We do not send live fish by courier or freight. Stock arrives in batches, so ring 0420 484 584 to check whether one is in.

An identification photograph, not a photograph of the individual fish currently in our tanks.

What it is

Salarias fasciatus, also sold as the jewelled rockskipper, sailfin blenny or algae blenny. Mottled brown and cream, a blunt frog-like head, eyes raised on top of it, and a habit of perching on rock and glass rather than swimming. Adults reach about 13 cm, which is larger than most people expect, and they want around 300 litres.

That volume figure surprises people, because a lawnmower blenny is usually recommended for small tanks. The size is not really about swimming room. It is about how much algae the tank grows, which is the whole of the problem with this fish.

The one thing that decides whether it lives

A lawnmower blenny eats film algae and short turf algae, scraped off rock and glass with a comb-like mouth, more or less continuously through the day. That is its entire diet in a tank. It is a grazer with a small stomach and no reserves, and it needs to graze constantly.

So the tank has to be producing algae faster than one fish can eat it, every day. A mature tank with a film problem does that easily. A clean, well-run, six-week-old tank does not, and a blenny put into one will graze it spotless in a fortnight and then slowly starve over the following months. It happens gradually enough that people do not connect the two, and the fish is usually described as having "just wasted away".

The practical rule is the opposite of how the fish is normally sold: buy a lawnmower blenny to fix an algae problem you already have, not to prevent one you might get. If your glass needs scraping weekly, the tank will support one. If it does not, the tank will not.

What it will not eat

It will not clear hair algae. This is the most common disappointment, and it is worth being blunt about because the fish is often sold as a general algae solution. Long hair algae, bryopsis and green sheet algae are not what that mouth is built for. It takes film and short turf, and it will ignore a hair algae outbreak growing next to it.

It also does not readily switch to prepared food. Some individuals will eventually take dried algae, and it is worth offering, but you cannot count on it and you should not buy one on the assumption that you can feed it if the algae runs out.

Feeding one through a lean patch

Dried algae sheets clipped in the tank are the thing that works, offered daily and left long enough for a grazer to find them. Nori and the marine algae sheets in our fish food range are what to use. Sinking herbivore pellets are worth trying alongside. Treat all of it as a supplement to a tank that already grows algae, not as a substitute for one.

Keeping one

It needs caves. Like all blennies it wedges itself into a hole in the rock with just the head showing. A tank with no crevice it can fit into is a tank where it hides behind equipment instead of grazing where you can see it.

One per tank. Peaceful toward other fish and territorial toward its own kind and toward other blennies competing for the same perch. Two in anything but a very large tank will fight.

It jumps and it perches high. Rockskipper is a fair name. A lid or a mesh cover is sensible, and so is checking that overflow boxes and returns do not give it a route out.

Reef safe. It does not eat coral. It will rest on it, and a blenny that repeatedly parks on the same LPS head can irritate it, which is worth watching but is not a reason to avoid the fish.

Common questions

Will it keep my glass clean?

It will keep the film down noticeably, and in a tank with enough algae to sustain it you will scrape less often. It will not replace a magnet cleaner, and it does not touch the corners or the silicone seams.

Can I keep one in a nano tank?

Not comfortably. A 13 cm grazer in a small tank runs out of food quickly, which is the same starvation problem in a shorter timeframe. For a small tank a midas or bicolour blenny is the better choice, and for algae in a nano the answer is usually a cleanup crew rather than a fish.

Is it the same as a sailfin blenny or an algae blenny?

Those are trade names for the same fish. Be aware that "algae blenny" is also used loosely for several smaller Salarias and Atrosalarias species with different adult sizes, so check the scientific name if the size matters to you.

Will it get on with my other fish?

Almost always. It ignores fish that are not blennies and is ignored in return. The exceptions are other grazers competing for the same food, so a lawnmower blenny plus a tang plus an urchin in one tank is more competition for the same algae than most tanks can feed.

If this is not the right fish

If you want a grazer and your tank is too clean to feed one, stock invertebrates instead. Trochus snails and an urchin do the same job without the starvation risk, because you can add a few at a time and match the numbers to the algae. That is covered on the cleanup crew page.

If you want a blenny for its behaviour rather than for algae, the midas blenny swims in open water and feeds on plankton, so it takes prepared food readily and does not depend on an established tank. See the blennies we stock, or the full marine fish range.

To check whether a lawnmower blenny is in, ring 0420 484 584. In-store purchase and pickup only.