If you only have time for the short answer: keep a single chromis in any tank from 30 gallons up, and do not attempt a group in anything under 125 gallons or 6 feet in length. Three to five chromis in a typical 75-gallon will collapse to one or two survivors within eighteen months in roughly seven out of ten cases. Seven or more chromis in a 180-gallon-plus tank with proper sightline breaks is the only stocking that gives you a real shot at the shimmering group you want.
That is the quick-answer block. Most readers can stop here. If you want to know why and how to set up the tank so the group actually holds together, keep reading. The rest of this guide unpacks the footprint geometry, the aquascape rules, and the bioload math that determine whether your chromis group survives or unravels.

Why 'gallons per fish' is the wrong question

The hobby's default stocking math, one inch of fish per gallon, or ten gallons per inch of fish for marine species, was built for solitary or weakly territorial fish. It does not work for chromis, and applying it gets people in trouble. Two chromis in a 30-gallon cube will fight more than four chromis in a 75-gallon long tank. The variable that matters is not volume; it is footprint.
Chromis defend territory along a horizontal axis. They are pelagic mid-water swimmers in the wild, cruising along reef shoulders for plankton, and their territorial behavior in captivity mirrors that geometry. A tall tank with a small footprint gives them no usable territorial real estate, even if the gallon count looks generous on paper. A long, shallow tank gives them everything they need at a fraction of the volume.
Reframe the question. Instead of 'how many chromis fit in seventy-five gallons,' ask 'how much footprint do I have, and how many chromis fit on that footprint with appropriate buffer zones.' Footprint plus aquascape plus group size, in that order, determines outcomes.

The footprint formula

Our internal rule of thumb, refined over years of customer stocking advice, is six to eight inches of tank length per chromis once you are past the critical-mass threshold. For a group of seven, that is forty-two to fifty-six inches of length, which lines up neatly with the six-foot (72-inch) tank we recommend as the realistic minimum for a stable group.
Width is the underrated dimension. A tank narrower than eighteen inches forces chromis into a near-linear arrangement, which collapses territories into a single row of contested zones. Eighteen inches of width allows fish to occupy multiple territorial pockets, front, middle, and back of the tank, instead of fighting along a single line. Twenty-four inches is better still.
Height is almost wasted volume for chromis. They use the middle-to-upper third of the water column for cruising and feeding, occasionally dropping to mid-level rockwork for refuge. The bottom six to eight inches of a tall tank may as well not exist from a chromis perspective. We tell customers shopping for a chromis-friendly system to prioritize length and width and not pay extra for a 24-inch-tall display when a 20-inch will do.

Tank size recommendations by group goal

Single chromis (any species)

Minimum tank: 30 gallons, roughly 36 inches long by 18 wide by 16 tall. A single chromis is essentially bulletproof. It will not fight anyone, will live eight to fifteen years, and will integrate smoothly with corals and most peaceful tankmates. If your tank is under 125 gallons and you want a chromis in it, this is the stocking we recommend.
Going below 30 gallons is workable for nano-tank specialists but tight. A 20-gallon long has the footprint to hold a single chromis comfortably, but you lose redundancy if other tankmates need space too. Check our nano fish selection for chromis-compatible smaller alternatives if your system is sub-30.

Bonded pair (rare, usually post-collapse)

Minimum tank: 55 gallons, ideally with a 48-inch length and 18 inches of width. Bonded pairs are usually the accidental survivors of a larger group rather than something you can engineer. If you have one, usually a male and female that have started defending a substrate patch together, they can live together for years.

Group of 3-5 chromis (high failure rate)

Minimum tank for the attempt: 75 gallons (48L × 18W × 20H). This is the most popular chromis stocking and our least-recommended. Failure rate at eighteen months sits above seventy percent based on the data we have aggregated. The math is brutal: with only three to five fish, the dominant has too few targets, and aggression concentrates on one or two subordinates until they die.
If you have already stocked this way and the group looks fine at month two, do not assume you are the exception. The collapse arc is described in detail in our chromis aggression timeline. Most reefers are still in the honeymoon phase when they ask us for advice.

Group of 5-7 chromis (moderate stability)

Minimum tank: 125 gallons, 72 inches by 18 inches by 22 inches. The six-foot length is the unlock. Six feet of length gives you roughly 2.5 territorial zones for chromis to disperse across, which is enough to keep aggression diffuse. A five-foot tank gives you about 1.5 zones, not enough. Survival rates jump significantly between 60 inches and 72 inches of length; we have seen this consistently across customer tanks.

Group of 7-12 chromis (best long-term stability)

Minimum tank: 180 gallons or larger, 72 inches by 24 inches by 24 inches at a minimum. This is where aggression genuinely diffuses across the group. The dominant fish cannot keep tabs on ten subordinates, so a stable pecking order forms instead of a death spiral. Stocking at this density also gives you margin, if you lose one or two fish during quarantine or settle-in, you still have enough critical mass to retain group dynamics.
This is the only chromis stocking we recommend for hobbyists who specifically want the 'wall of chromis' look using a Chromis viridis type. If you do not have 180 gallons of horizontal real estate, choose a peaceful species like Chromis vanderbilti or Chromis retrofasciata, see our best chromis species guide, or substitute anthias for the real schooling behavior.

Group of 15+ chromis (true 'wall of chromis' effect)

Minimum tank: 240 gallons or larger with at least eight feet of length. At this density, the visual effect rivals what you would see on a wild reef. Bioload management becomes a serious project, chromis are light individual feeders but the cumulative waste from fifteen fish requires an SPS-grade husbandry approach. Plan for oversized protein skimmers, a healthy refugium, and disciplined filter sock maintenance.

Why length matters more than depth

Chromis defend territories along the horizontal, that is the single most important geometric fact about this genus in captivity. A 5-foot tank gives roughly 1.5 territories of separation between dominant and subordinate. A 6-foot tank gives roughly 2.5. The jump is non-linear because territories overlap; doubling the linear count slightly more than doubles the effective buffer.
Four-foot tanks, the most common 'beginner' size in the U.S. market, compress all chromis into a single territorial zone. There is no second zone for a subordinate to retreat into. Every fish is in line-of-sight of every other fish for twenty-four hours a day. This is why a group of five in a 75-gallon almost always collapses, while a group of five in a 125-gallon long has a meaningfully better chance even though gallon count is the typical headline number.
If you are budget-constrained and choosing between a tall 90-gallon and a long 90-gallon for chromis, take the long version every time. The extra height does nothing for these fish; the extra length does everything.

Aquascape design for chromis groups

Aquascape is the second-biggest lever after tank size. A perfectly sized tank with the wrong rockwork will still collapse a chromis group; a slightly undersized tank with deliberate sightline geometry can punch above its weight. The principles are straightforward.
Build two or three rockwork columns that block visual access from one end of the tank to the other. The goal is that no fish in the back-left territory can see the fish in the back-right territory without swimming around a structure. Live rock branches and arches are particularly useful, they create real visual breaks without consuming all your coral real estate. Hobbyists planning a build should browse our saltwater aquascaping materials with chromis territory in mind, not just coral placement.
Avoid flat 'wall' aquascapes that span the entire back glass, they look impressive but they create one shared territory along a long ledge, which guarantees daily confrontation. Leave an open swim corridor along the front pane of the tank where chromis can cruise without crossing defended zones. They are pelagic; they need linear space to feel comfortable.
Manage flow placement deliberately. Chromis love high-flow zones; if you create one obvious flow hotspot, every fish will fight to occupy it. Use two medium powerheads on opposite ends or a Vortech-style gyre to spread flow evenly. This is the single cheapest aggression intervention in chromis husbandry.

Bioload and filtration considerations

Individually, chromis are light feeders and modest waste producers, roughly 0.3 to 0.5 pounds of bioload-equivalent per adult fish. In groups of ten or more, that cumulative load becomes meaningful. Plan filtration capacity at 1.5 to 2 times your nominal tank volume; a protein skimmer rated for a 250-gallon tank on a 150-gallon system is appropriate, not overkill.
Chromis spawn frequently in captivity even without producing viable fry. The eggs go uneaten and decompose in your sump. Run a refugium to process the nutrient load and change filter socks every three to four days for chromis-heavy systems. Use reliable water testing equipment on a weekly cadence, a sudden nitrate spike can push the weakest fish in the hierarchy over the edge.

Common stocking mistakes

The most common stocking mistake we see, by a wide margin, is the classic five-chromis-in-a-75-gallon plan. It is the cheapest way to get a 'school' into a popular tank size, and it is also the configuration with the worst long-term outcome in our dataset. We recommend either dropping to one chromis or upsizing the tank before stocking.
Adding chromis last in the stocking order is another consistent error. Staggered introduction means the first chromis claims the whole tank and bullies any newcomer. Stock all your chromis simultaneously, ideally before or alongside any other community fish. And avoid the 'I'll upgrade later' plan, by the time you upgrade, the group has collapsed to one or two territorial survivors, and you cannot meaningfully add to it. Plan tank size for the stocking you actually want.

Quick reference table

Frequently Asked Questions

How many green chromis in a 75 gallon?

Realistically one to three long-term. Stocking five, the most popular plan, will collapse to one or two fish within eighteen months in roughly seventy percent of cases. If you want a stable group, upsize to a 125-gallon six-foot tank or choose a peaceful chromis species.

Can I keep chromis in a 30 gallon?

One chromis only. A 30-gallon long with a 36-inch footprint is fine for a single specimen. Do not attempt a pair or group in this volume, territories cannot meaningfully separate in that footprint.

What's the minimum tank size for blue chromis?

Chromis cyanea (Atlantic blue chromis) should be kept singly in a 40-gallon tank or larger. Groups are not recommended at any tank size, blue chromis are among the most aggressive species in the genus.

Can I add more chromis later?

Almost never works once the hierarchy is established. Newcomers are killed by resident dominants within two to three days. If you need to rebuild a group, your best path is to rehome the survivors, re-aquascape the tank, and restock with seven or more new chromis simultaneously.