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The green chromis, often sold as the blue-green chromis, is one of the best-known saltwater fish in the hobby. It is cheap, shimmering and active, and it looks best in a group over a reef. It is also surrounded by myths: that it is always peaceful, that it is bulletproof, and that it can live happily alone.
This green chromis care guide covers size, tank requirements, how many to keep, feeding, tank mates, lifespan and the problems that most often hit this fish.
Green Chromis Overview and Appearance
Photo: NasserHalaweh (CC BY-SA 4.0), via Wikimedia Commons.
The green chromis (Chromis viridis) is a damselfish from the Indo-Pacific and Red Sea. In the wild it hovers in large groups above branching corals such as Acropora and Pocillopora, darting into the branches when threatened. The body is a pale, shimmery blue-green, and the color looks brighter under blue actinic lighting. Many keepers search for “green chromis vs blue chromis”, and the answer is simple: in the trade the names are used for the same species, and the color you see depends on lighting and mood.
Size and Lifespan
Green chromis reach about 3 to 4 inches (7 to 10 cm) as adults. The green chromis lifespan in captivity is usually 5 to 8 years in a stable tank, though many are lost much earlier because of poor collection, shipping stress or aggression.
Green Chromis Tank Size and Setup
A single chromis does not do well. For a group of 3 to 5, the green chromis minimum tank size is about 55 gallons, and larger is better. Keeping a school in a 20 or 30 gallon tank usually ends with the dominant fish bullying the others to death.
Setup basics:
- Open water above the rock. Chromis spend their day swimming in the middle of the tank, so do not fill every inch with rock.
- Branching rock or coral for retreat. They need places to dive when stressed.
- Mature, cycled system. Follow our guide on how to set up a saltwater aquarium and finish cycling the tank first.
- Moderate to strong flow and good oxygenation.
- A lid. Chromis can jump when startled.
Photo: Lisa Bennett (CC BY 4.0), via Wikimedia Commons.
Water Parameters
- Temperature: 75-82°F (24-28°C)
- Salinity: specific gravity 1.023-1.026
- pH: 8.1-8.4
- Ammonia and nitrite: 0 ppm
- Nitrate: below 20 ppm
Chromis are often called hardy, but they react badly to swings in salinity and temperature. A reliable heater, a refractometer and a saltwater test kit keep things stable.
What Do Green Chromis Eat?
Green chromis are planktivores. In the wild they pick tiny drifting food from the water column. In the aquarium they eat almost anything small, but they do best with frequent small meals:
- Quality marine flakes or small pellets
- Frozen mysis shrimp and baby brine shrimp
- Enriched copepods or finely chopped meaty foods
- A little marine algae or spirulina
Feed two or three times a day in small amounts. Chromis that are fed once a day tend to lose condition slowly, and underfed fish bicker more. A good marine fish pellet plus frozen foods covers the basics.
Are Green Chromis Aggressive? Temperament and Tank Mates
Green chromis are usually described as peaceful, and that is mostly true in a large tank with a bigger school. In a small tank, or with only two fish, one chromis will often pick on the other. They are damselfish, after all.
How many green chromis should I keep? Either one in a small reef or 5 or more in a large tank. Avoid pairs. Odd numbers (5, 7, 9) spread aggression better than even numbers.
Good tank mates include:
- Clownfish. See our ocellaris clownfish care guide. In a large tank the two get along fine.
- Gobies, blennies and dartfish
- Royal gramma and other peaceful basslets
- Cardinalfish and anthias in large tanks
Avoid keeping them with large predators such as lionfish, groupers or big triggerfish, which eat them. Also avoid very aggressive tangs and dottybacks that harass a small school.
Also remember that chromis are sensitive to harsh meds, so quarantine new fish in a separate tank before adding them.
Breeding Green Chromis
Green chromis can spawn in a home aquarium, but raising the fry is difficult. A male prepares a nest on a rock or coral branch and the female lays small adhesive eggs. The male guards the eggs for a few days. The larvae are tiny and need live rotifers or similar foods, so most eggs are eaten by tank mates or filtered out. Breeding in a home tank is a curiosity, not a reliable project.
Common Health Issues
- Marine ich (cryptocaryon). Chromis are prone to ich after shipping stress. White dots, rapid breathing and scratching are the signs. See our guide on white spots on fish and treat in a hospital tank rather than in the display.
- Brooklynella and velvet. These parasites can kill a chromis quickly, so watch for slime, gasping and weight loss.
- Aggression injuries. Torn fins and missing scales usually mean the school is too small or the tank is too crowded.
- Poor collection and shipping. Many chromis are net- or cyanide-stressed. Choose fish that are actively eating before you buy.
Treat sick fish in a separate tank where you can control the dose. Our top 5 aquarium medications guide explains the options, though many freshwater meds are not suitable for saltwater or reef tanks.
How Hard Is Green Chromis Care?
Difficulty: easy to moderate. They are tolerant of small water changes and common foods, which is why they are so common as starter fish. The risks are buying too few fish, putting them in a tank that is too small, and failing to quarantine. A mature, cycled 55 gallon or larger tank with a school of five or more gives the best result.
Photo: Debra Baker (CC BY 4.0), via Wikimedia Commons.
Green Chromis FAQ
Are green chromis aggressive?
They are mostly peaceful in a school of 5 or more in a large tank. In a small tank or as a pair they can be aggressive toward each other.
Can green chromis live with clownfish?
Yes, in tanks of 55 gallons or larger. Give the clownfish its own space and plenty of rock.
What is the minimum tank size for green chromis?
About 55 gallons for a small group. A single fish can live in a smaller reef, but a school looks and behaves best in a larger tank.
What do green chromis eat?
Small meaty foods, flakes, pellets and frozen mysis, offered two or three times a day.
How long do green chromis live?
Often 5 to 8 years in stable conditions.
Are green chromis reef safe?
Yes. They do not bother corals or most invertebrates.
Final Thoughts
Green chromis are a lovely, affordable way to add life to the middle of a reef, as long as you give them enough room and enough company. Buy a small group, feed several times a day, quarantine them and keep the water stable, and they will reward you with years of shimmering activity.
Do you keep a school of chromis? Share your tips in the comments.
Disclaimer: This article is general information, not veterinary advice. For serious or persistent problems, consult an aquatic veterinarian.