Saltwater Fish

Green Chromis

Chromis viridis

Active blue-green shoaler for a peaceful reefBeginner

Lifespan: 8-15 years
Size: 3-4 in
Social needs: Shoaling; keep 6+ together, added at the same time
Time: Daily feeding; weekly testing/changes
Cost: High
Diet: Omnivore (mysis, brine, marine flake/pellet)

Overview

What is this animal?

The Green Chromis is an iridescent blue-green damselfish that schools in the open water of a reef.
Hardy, inexpensive, and active, it adds constant movement, and unlike most damsels it is relatively peaceful, which makes it a reasonable beginner shoaling species.

They are among the tougher marine fish available and tolerate minor beginner mistakes within a cycled tank.
A well-fed group moving in unison across the tank is a striking display.

Temperament

Peaceful by damselfish standards and reef-safe, Green Chromis school together in open water and ignore corals.
However, as they mature a hierarchy forms and dominant fish often pick off subordinates one by one until only one or two remain.

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Counter this by stocking a group of six or more at once in a roomy tank.
They mix well with other peaceful community fish and add coordinated open-water movement.

A good fit for

Beginners wanting an active group
Larger reef and FOWLR tanks
Keepers on a tighter livestock budget
Adding open-water movement

Common mistakes to avoid

Buying too few - they cull down to one
Adding them piecemeal instead of together
Undersized tank for an active school
Aggression as the group matures

Enclosure

Minimum 79 gallons, for a group of 6

Cannot be kept alone. The minimum is 6, and the space above is for that group.

Keep a group of six or more in a cycled tank of at least 80 gal, so the school has swimming room and the weaker fish have space to get away.
Maintain salinity 1.024-1.026, temperature 75-80°F, pH 8.1-8.4, and nitrate low, with the moderate-to-strong flow they prefer.

Provide open swimming space plus live rock for refuge.
A larger tank and bigger group help spread out the aggression that develops as they mature and keep the schooling behaviour intact over the long term.

Check your enclosure

Supplies

Setup

Tank, stand and sump, about 80-90 US gallons
Return pump and powerheads
Protein skimmer
Reef lighting
Rock and sand

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Salt mix, refractometer and test kits
Quarantine tank

Ongoing

Marine flake or small pellet
Frozen mysis shrimp
Frozen brine shrimp
Frozen cyclops or copepods
Algae-based marine food

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Small thawing cup and strainer
Turkey baster for spreading food in the current

Safe substrate: Aragonite sand · Dry or cured live rock

Work out the cost

Care routine

Daily

Check salinity, temperature and flow
Feed
Head-count and look everyone over
Top up evaporation with fresh water

Every few days

Service the skimmer and filter socksevery 3 days

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Weekly

Clean the glass
Count the group
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 group and aggression review, every 3 months

Tools

Is your enclosure big enough?Check your measurements against the recommended size What will it cost?Setup, monthly and over its whole life, itemised Is this ammonia reading dangerous?Works out the toxic part from your pH and temperature

Care in depth

Diet

Omnivore (mysis, brine, marine flake/pellet)

Green Chromis are omnivores.
Offer a marine flake or pellet staple plus frozen mysis and brine shrimp, fed once or twice daily.
In the wild they pick zooplankton from the water column, so they appreciate small foods that drift in the current.

They feed eagerly and rarely need coaxing, which makes them easy to keep well-fed in a busy tank.
A varied diet maintains their blue-green sheen and supports the group's condition and activity.

Health

Signs of good health, and the common problems

Chromis are hardy and disease-resistant, tolerating minor parameter swings better than most marine fish.
The main issue is stress-driven aggression within the group, which can lead to injuries, bacterial infections, and a dwindling school.

Quarantine new fish and keep nitrate low; marine ich is the usual risk after poor quarantine.
Watch for nipped fins and isolated, hiding individuals as signs of in-group bullying.
A large enough group and tank keeps stress and disease low.

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.

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.

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.

safe

Dry or cured live rockDry rock avoids importing pests but takes longer to mature. Cured rock is faster and riskier.

safe

With 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 in deep that it turns over itself. That is a different thing from a denitrifying deep sand bed (4 in 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 caveat

Uncured live rockIt will cycle the tank for you, and it will also bring in whatever was living in it.

with a caveat

Bare 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 in 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 caveat

Never

Silica play sandNot the marine standard, and commonly blamed for feeding diatom blooms in a new tank.

never

Any rock or decor of unknown originMetals and phosphate leach for months and are almost impossible to trace once the tank is running.

never

This is our general list for saltwater fish. We have not built one specific to Damselfish yet.

Top resources for more info

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