Saltwater Fish · Blennies
Bicolor Blenny
Ecsenius bicolor
Two-toned charmer with a big personalityBeginner
- Lifespan
- 4-6 years
- Size
- 3-4 in8-10 cm
- Social needs
- One per tank unless a bonded pair
- Time
- Low
- Cost
- High
- Diet
- Omnivore, algae-leaning
Overview
What is this animal?
- The Bicolor Blenny is a hardy, comical fish, dusky blue at the front and orange at the back, with expressive eyes and a habit of perching and peeking from rocks.
- Its personality and toughness make it a popular beginner marine.
- It is reef-safe and helps graze film algae, though it needs supplemental feeding once natural algae thins.
- Easy to keep and full of character, it suits new and experienced reef keepers alike.
Temperament
- Generally peaceful but territorial toward other blennies and sometimes similar small fish, occasionally nipping at tankmates that invade its space.
- Keep one per tank unless you have a bonded pair.
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- It is reef-safe with corals and invertebrates, though the rare individual may nip coral polyps if underfed.
- Mostly it is an entertaining, characterful fish that perches openly and interacts with its keeper.
A good fit for
- Beginners wanting a hardy, characterful fish
- Reef tanks with film algae to graze
- Keepers who will supplement with nori
- Peaceful community reef systems
Common mistakes to avoid
- Starves in tanks lacking enough algae
- Known jumper; needs a secure lid
- Two blennies together usually fight
- Rare underfed individuals may nip corals
Enclosure
Minimum 79 gallons300 litres
One of its kind per tank; other compatible species can share it. One per tank unless a bonded pair
- House in a reef tank of at least 80 gallons300 L with plenty of live rock providing algae, perches, and a hole or crevice it can claim as a home base.
- A mature tank with algae film suits it well.
- Maintain temperature 75-81°F24-27°C, salinity 1.024-1.026, pH 8.1-8.4, and ammonia and nitrite at zero.
- A secure rock retreat gives this perching fish confidence and frequent visibility.
Supplies
Setup
- Tank, stand and sump, about 80-90 US gallons300-340 litres
- Return pump and powerheads
- Protein skimmer
- Reef lighting
- Rock and sand
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- Salt mix, refractometer and test kits
- Quarantine tank
Ongoing
- Dried marine algae (nori) sheets
- Magnetic or suction veg clip
- Spirulina flake and algae pellets
- Frozen mysis shrimp
- Frozen enriched brine shrimp
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- Marine flake or small pellet
- Small thawing cup and strainer
Safe substrate: Aragonite sand · Dry or cured live rock
Care routine
Daily
- Check salinity, temperature and flow
- Feed
- Head-count and look everyone over
- Top up evaporation with fresh water
Every few days
- Offer nori on a clipevery 2 days
- Service the skimmer and filter socksevery 3 days
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Weekly
- Check the belly profile
- Clean the glass
- Test the water
- Water change
Monthly
- Calibrate the refractometer
- Check salt, RO/DI and reagents
Less often
- Deep-clean pumps and powerheadsevery 3 months
- Replace RO/DI filter mediaevery 6 months
Health review: Quarterly belly-profile review, every 3 months
Tools
Care in depth
Diet
Omnivore, algae-leaning
- An algae-leaning omnivore that grazes film algae and diatoms but also takes meaty foods.
- Offer dried nori, spirulina flakes, algae pellets, and frozen mysis and brine shrimp one to two times daily.
- A varied diet weighted toward vegetable matter keeps it healthy and reduces any temptation to nip corals.
- Supplement well once natural tank algae is grazed down to prevent a hollow belly.
Health
Signs of good health, and the common problems
- Hardy and disease-resistant relative to many marine fish, though quarantine and stable water are still sensible.
- Most losses come from starvation in algae-poor tanks or from jumping rather than from illness.
- Watch the belly profile and appetite; a pinched stomach signals the need for more food.
- Provide a secure lid, as bicolor blennies are capable jumpers given any open gap.
Is this urgent?
6 warning signs and how fast to act, 3 of them right away
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. Ammonia is directly toxic, and at reef pH more of it is in the toxic form, so a marine system has less margin than a freshwater one. Nitrite is far less toxic in seawater, but any reading in a stocked tank means the biological filter is not keeping up.
NowTemperature 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.
NowRapid 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.
NowToday
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.
TodayCorals 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.
TodayA 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.
TodayQuarantine 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.
Bedding and substrate
2 safe, 3 with a caveat, 2 to avoid
In a marine system the rock and the sand are life support, and how you start them decides the first six months.
Safe
Aragonite sandThe standard marine substrate, and it hosts the biology. It does not meaningfully hold pH or alkalinity at normal reef pH, so test and dose for those regardless.
safeDry or cured live rockDry rock avoids importing pests but takes longer to mature. Cured rock is faster and riskier.
safeWith a caveat
Deep sand bedsPowerful nutrient processing when they work and a sulphide release when they are disturbed. Choose shallow or deep deliberately, not by accident. A sand-sifting or burrowing goby needs a fine bed about 2 to 3 in5 to 8 cm deep that it turns over itself. That is a different thing from a denitrifying deep sand bed (4 in10 cm or more, left undisturbed), and the two do not mix: a sifting goby disturbs the layers a deep sand bed depends on and eats the life in it.
with a caveatUncured live rockIt will cycle the tank for you, and it will also bring in whatever was living in it.
with a caveatBare bottomWorkable in high-flow systems for fish that neither sift nor burrow, but it removes a large part of the biological buffer. Wrong for watchman gobies and other gobies that burrow or sift, which need a fine sand bed at least 2 in5 cm deep to dig and feed in, and for mandarins and other copepod-eaters, which need sand as well as mature rock for their food to breed in.
with a caveatNever
Silica play sandNot the marine standard, and commonly blamed for feeding diatom blooms in a new tank.
neverAny rock or decor of unknown originMetals and phosphate leach for months and are almost impossible to trace once the tank is running.
neverThis is our general list for saltwater fish. We have not built one specific to Blennies yet.
Top resources for more info
Independent creators and organisations we would send a friend to. We have no affiliation with any of them, and nothing here is sponsored. Links checked 2026-10-22.
YouTubers
HumbleFish & ReefSaltwater fish disease, quarantine and treatment (Humble.Fish forum)BRStvReef tank setup, fish health and quarantine series (run by a retailer)FishOfHex AquaticsBeginner saltwater guides, avoiding common mistakes and tank disasters
Sites
Keep going
Blennies, side by side
Same axes, same care guides: the honest way to choose between types.
