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.
Housing
Keep a group in a cycled tank of 110L (30 gal) minimum, with larger volumes strongly preferred so the school has swimming room.
Maintain salinity 1.024-1.026, temperature 24-27°C (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.
Diet
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
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.
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.
Counter this by stocking an odd-numbered group of five 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
Is your enclosure big enough?
Minimum volume110 litres (29 US gallons)
CompanyCannot be kept alone. The minimum is 3, and the space above is for that group.
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.
Beginner
Beginner
Beginner
Beginner
Beginner
Beginner