Blackbreast cardinalfish
Xeniamia atrithorax
This is a tiny deepwater cardinalfish that was only described in 2016, and it stays around 3 cm long max. The cool calling-card is the dark "blackbreast" patch on the chest area and the fact that the males mouthbrood eggs like other cardinalfish, even though it comes from way deeper water than the usual reef tank cardinals.
The Blackbreast cardinalfish features a striking dark band across its body and distinctive white to light blue spots on its fins, enhancing its visual appeal.
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Quick Facts
Size
3 cm SL
Temperament
Peaceful
Difficulty
Expert
Min Tank Size
20 gallons
Lifespan
unknown
Origin
Western Pacific (South China Sea - Vietnam and Taiwan)
Diet
Carnivore/planktivore - likely tiny crustaceans and zooplankton (offer copepods, enriched baby brine, small mysis)
Water Parameters
22-26°C
8-8.4
8-12 dGH
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Calculate heater sizeCare Notes
- Give them a mature, quiet reef tank with lots of branching rock or coral-like cover - they hover and spook easily, and they do way better when they have tight spots to duck into.
- Keep salinity steady around 1.025-1.026 and temp in the 24-26 C (75-79 F) range; they are not forgiving if your salinity swings from top-offs or sloppy water changes.
- They are dusk feeders - hit them with small meaty foods (mysis, finely chopped shrimp, enriched brine, copepods) after lights down or in low flow so the food actually reaches them.
- Feed small portions 2-3 times a day instead of one big dump; they are slow pickers and will lose out to fast, pushy fish at mealtime.
- Skip housing them with aggressive dottybacks, big wrasses, or any boisterous feeder that will harass them or outcompete them; they do best with other calm nano reef fish and peaceful gobies/blennies.
- If you keep more than one, watch for bullying in tight tanks - a bonded pair is great, but random groups can turn into one fish getting pinned in a corner and wasting away.
- Breeding is classic mouthbrooder stuff: the male will hold the eggs and stop eating for a while, so do not freak out and try to force-feed; keep stress low and keep tankmates from pestering him.
Compatibility
Good Tankmates
- Other Blackbreast cardinalfish (Xeniamia atrithorax) - honestly they look and act best in a small group, and they are way more relaxed when they have their own kind around (just give them rockwork to hover under).
- Ocellaris or percula clownfish - typical mellow clowns that stay by their spot and do not hassle cardinals much.
- Firefish (Nemateleotris spp.) - peaceful, hangs in the water column like the cardinals do, and tends to keep to itself as long as the tank is not full of bullies.
- Small, chill gobies (neon goby, clown goby, watchman types) - they do their own bottom or perch thing and do not compete much with cardinals.
- Blennies with a good attitude (tailspot blenny, bicolor blenny in most setups) - generally ignore cardinals, just make sure everyone is getting food since cardinals can be slow at mealtime.
- Peaceful wrasses like a flasher or fairy wrasse - active but usually not mean, and they do not typically pick on cardinals if the tank is sized right and has hiding spots.
Avoid
- Dottybacks (especially orchid dottyback, and definitely the more aggressive ones) - they can be little terrors in the rockwork and will harass shy fish that like to hover and hide.
- Hawkfish (flame hawkfish and similar) - they can be pushy, and if the cardinal is small they can get treated like a snack or at least bullied off the good perches.
- Aggressive damsels (domino, three-stripe, etc.) - constant in-your-face stress, and cardinals really do not thrive with that kind of chaos.
- Big semi-aggressive fish like triggers or larger dottyback-style predators - anything that views a small, slow, hover-y fish as food, or just likes to throw its weight around.
Where they come from
Blackbreast cardinalfish (Xeniamia atrithorax) are a small, cryptic cardinalfish from Indonesia. You do not see them cruising out in the open like banggais. They hang tight to shady reef structure and hover in place, picking at tiny food drifting by.
They are one of those fish that look "easy" because they are small and calm, but they are picky in a very cardinalfish way: shipping stress, feeding response, and being outcompeted can make or break them.
Setting up their tank
Think shaded caves, overhangs, and calm pockets of water. If you set up a bright, bare, high-flow SPS raceway, they will spend all day wedged behind a powerhead and come out only when the lights are off.
- Tank size: I would not do them in less than 20-30 gallons, and bigger is easier if you want a small group.
- Aquascape: rock with lots of crevices and at least a couple real "hangout" spots (a cave with a ceiling, not just a tunnel).
- Lighting: they appreciate dim areas. If your tank is bright, add overhangs or macro/branching coral to break up the light.
- Flow: moderate overall, but make sure there are low-flow zones where they can hover without fighting the current.
- Filtration: stable, mature reef-style setup. They do better once a tank has some life in it (pods and general microfauna help).
This is an expert fish mostly because of feeding and stress. I strongly recommend quarantine, and I would only add them to a tank that is already stable and boring (in a good way). New tanks and brand-new rock are a rough start.
For parameters, aim for steady reef numbers more than chasing a magic value. Consistency matters: salinity that does not swing, temperature that does not bounce day to day, and good oxygenation. They are not fans of being on the edge of hypoxia at night.
What to feed them
These guys are small-mouthed, deliberate feeders. In my tanks, the biggest hurdle was getting them confidently eating prepared foods while faster fish tried to vacuum everything up.
- Best starters: live baby brine (enriched), copepods, live blackworms (if you use them carefully), or very small live foods to trigger a response.
- Frozen they usually accept with time: mysis (smaller pieces), enriched brine, chopped krill, calanus, cyclops, roe, and finely chopped seafood.
- Prepared: some will learn pellets, but do not count on it early. Tiny sinking pellets can work once they are settled.
Target feeding helps a lot. I use a turkey baster or pipette and gently "cloud" food right in their hovering spot. Feed the fast fish on the other side first, then feed the cardinals in peace.
Multiple small feedings beat one big dump. If you can do 2-4 lighter feedings, you will see better weight and less bullying at meal time.
How they behave and who they get along with
They are calm and a little shy, especially at first. Once settled, they hover in the same few spots and watch you like tiny drones.
- Good tankmates: other peaceful reef fish that do not rush food (gobies, blennies, small wrasses that are not overly aggressive, calm clowns, firefish with enough space).
- Use caution: dottybacks, hawkfish, big wrasses, aggressive clowns, and anything that treats small fish as snacks.
- Competition issue: even "peaceful" fish like anthias and chromis can outcompete them at feeding time.
If you want more than one, add them together and give them multiple bolt-holes. With a single fish, they can be timid for a long time. In a small group, they are often bolder, but you need space and enough hiding spots to prevent one from getting picked on.
Breeding tips
Like a lot of cardinalfish, they are mouthbrooders. If you keep a compatible pair and they are well-fed, you may see the male stop eating and hold a clutch. He will look like he is chewing or keeping his mouth slightly open, and he will hang back even more than usual.
- Give the holding male peace: low stress, no net-chasing, and keep boisterous fish away from his favorite cave.
- Do not try to "test" if he is holding by startling him. He can spit the eggs or swallow them.
- If you want to raise fry, you will need a separate rearing setup and tiny foods ready (rotifers/copepod nauplii, then baby brine).
Most losses I have seen around breeding attempts come from the male not eating for a long stretch and then getting bullied or stressed. A calm tank and frequent small feedings before he holds makes a difference.
Common problems to watch for
- Refusing food after purchase: very common. Start with live or very small frozen and feed in low-flow shaded areas.
- Getting skinny while "eating": they may grab food but lose the battle to faster fish. Watch belly shape over weeks, not just one feeding.
- Hiding nonstop: usually too much light/flow, not enough cover, or too much commotion from tankmates.
- Shipping stress and disease: they can come in rough. Quarantine and observe for typical marine issues like ich/velvet and bacterial infections.
- Jumping: less notorious than some fish, but still possible if startled. A lid is cheap insurance.
If a new blackbreast cardinalfish is breathing fast, staying pinned in a corner, and not responding to food for days, do not just wait it out. These fish can go downhill quietly. Check oxygenation, temperature, ammonia, and consider that it may be dealing with velvet or another fast-moving problem.
If you set them up with shade, calm flow, and you take feeding seriously (small foods, targeted, and frequent), they become one of those fish you end up staring at more than your "show" fish. They are subtle, but really rewarding once you crack the code.
Similar Species
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