Bicolor Blenny

Ecsenius bicolor

The two-tone nano blenny with attitude — an eager algae-grazer that may nip corals if underfed.

Difficulty: BeginnerReef-safe community fishReef-safe

The bicolor blenny (Ecsenius bicolor) is a nano-reef favourite with a split personality to match its two-tone look — dark blue-grey up front, orange behind — perching in rock holes, darting out to graze, and eyeing you with froggy eyes. It's a hardy, entertaining algae-grazer, with two honest caveats: it can turn to nipping corals when algae runs low, and it's territorial toward other blennies.

Good to know: May pick at coral polyps/clam mantles if algae runs low; territorial toward other blennies.

📊 What keepers actually ask: Loved for personality; the recurring issues are "it started nipping my corals" (feed it) and blenny-vs-blenny squabbles.

Care at a glance

Adult size	~4 in.
Minimum tank	10+ gallons with plenty of live rock and holes to perch in.
Temperature	75–82°F (24–28°C).
pH	8.1–8.4.
Specific gravity	1.023–1.026 (reef-standard ≈35 ppt).
Diet	Herbivore-leaning omnivore — grazes film and hair algae off rock; supplement with nori and prepared foods, especially in a clean tank.
Temperament	Characterful and generally peaceful to other species, but territorial toward other blennies. Keep one blenny per (small) tank.
Reef-safe	Mostly — it grazes algae, but a bicolor can start nipping coral polyps or clam mantles when its algae supply runs low. Keep it fed.
Natural range	Indo-Pacific.

Beginner-friendly, done right. A hardy, cheeky nano fish and a good algae-grazer — but with more of a "reef-safe with caution" streak than the tailspot blenny. Bicolors are known to shift from algae to nipping coral polyps or clam mantles when microfauna and algae run short, so keep the tank algae-rich and supplement with nori. They're also territorial toward other blennies (and blenny-shaped fish), so one per small tank. Quarantine before adding.

Planning your stock list? Before you buy, run your tank and wish-list through our free stocking & tank-size calculator — it checks this fish's minimum tank and shoal size and flags any aggression clash, so you don't overstock.

The fish store gets these wrong

Common myths, kindly corrected — no shame for having believed them.

"Blennies never touch corals." The bicolor can — when algae runs low it may nip coral polyps or clam mantles. A well-fed, algae-rich tank prevents most of it.
"I can keep two blennies in my nano." Usually not — blennies are territorial toward each other; one per small tank is the rule.

Before you buy

An algae-rich tank (or a plan to feed nori) to keep it off corals.
Rock with holes and perches to claim.
One blenny per (small) tank; quarantine before adding.

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:

White spots, flashing or rapid breathing — quarantine and treat.
Not grazing/eating, constant hiding, or thinning — stress or illness.
Clamped fins or cloudy eyes — a water-quality problem.

Never

Never keep it in a clean tank with no algae and no feeding — it'll turn to corals.
Never keep two blennies in a small tank.
Never skip quarantine.

Common bicolor blenny questions

Is the bicolor blenny reef-safe?

Mostly — it's primarily an algae grazer, but bicolors are known to nip coral polyps or clam mantles when their algae and microfauna supply runs low. Keep the tank algae-rich and supplement with nori and it's usually well-behaved.

What do bicolor blennies eat?

Chiefly film and hair algae grazed off the rock, plus prepared foods. In a clean tank, offer dried seaweed (nori) and small meaty/herbivore foods so it doesn't turn to your corals.

Can I keep two blennies together?

Generally no — blennies are territorial toward each other and similar-shaped fish. One blenny per (small) tank is the safe rule; only very large tanks with lots of territory can sometimes support more.

How big do bicolor blennies get and what tank do they need?

Around 4 inches, and they're happy in 10 gallons and up with plenty of live rock full of holes to perch and hide in.

Bicolor or tailspot blenny — which should I get?

The tailspot blenny is the more peaceful, more reliably reef-safe grazer; the bicolor has more personality but is a bit nippier and more territorial. For a coral-heavy nano, many keepers prefer the tailspot.

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:

ReefBay — Complete Blenny Care Guide — coral-nipping caveat, tank, diet
AquariumStoreDepot — Types of Blennies — temperament, algae grazing

Written and fact-checked by The Coralkin Team against marine-aquarist and veterinary sources — if your bicolor 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 →

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