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Pajama Cardinalfish
Saltwater Fish · Cardinalfish

Pajama Cardinalfish

Sphaeramia nematoptera

Peaceful nocturnal cardinalfish that lives in groupsBeginner

Lifespan
4-5 years
Size
3 in
Social needs
Shoaling; keep in a small group
Time
Daily feeding; weekly testing/changes
Cost
High
Diet
Carnivore (mysis, brine, frozen, pellets)

Overview

What is this animal?

  • The Pajama Cardinalfish has a silver-yellow face, a dark band across the body, and a polka-dotted rear, giving it a mismatched "pyjama" look.
  • It is peaceful, hardy, and reef-safe, and like most cardinalfish it lives in groups, hovering together near branching cover during the day.
  • Naturally nocturnal, it becomes most active at dusk but adapts to feed during the day in aquariums.
  • Its slow, gentle drifting makes it a distinctive addition that contrasts with more active swimmers.

Temperament

  • Peaceful, slow-moving, and reef-safe, the Pajama Cardinalfish is harmless to corals and most invertebrates, though very small shrimp may be eaten.
  • It does best kept in a small group rather than alone.
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  • Lone fish tend to be reclusive, while a group of three or more drifts confidently in the open.
  • Pair them only with calm tankmates, as fin-nippers and aggressive fish will keep them hidden and stressed.

A good fit for

  • Beginners wanting a peaceful group
  • Reef community tanks
  • Keepers who like calm, hovering groups
  • Dusk-active display tanks

Common mistakes to avoid

  • Keeping just one (becomes reclusive)
  • Fast tankmates stealing their food
  • Pairing with aggressive or fin-nipping fish
  • Insufficient hiding cover causing stress

Enclosure

Minimum 160 gallons, for a group of 3

Cannot be kept alone. The minimum is 3, and the space above is for that group.

  • A cycled tank of at least 159 gal lets you keep a small group of three or more among live rock and branching coral cover.
  • Maintain salinity 1.024-1.026, temperature 75-80°F, pH 8.1-8.4, and nitrate low.
  • Moderate flow and a covered top are best.
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  • Provide overhangs and branching structure where the group can rest and feel secure.
  • Stable, clean water and ample cover keep these gentle fish confident and out in view during daylight hours.

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

  • Frozen mysis shrimp
  • Frozen enriched brine shrimp
  • Frozen or live copepods
  • Small marine pellets
  • Vitamin or HUFA soaking supplement
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  • Turkey baster for target-feeding
  • Small thawing cup and strainer

Safe substrate: Aragonite sand · Dry or cured live rock

Work out the cost

Care routine

Daily

  • Check salinity, temperature and flow
  • Head-count and look everyone over
  • Small feeds once or twice a day, and watch each fish eat
  • 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
  • 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 condition review, every 3 months

Tools

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

Care in depth

Diet

Carnivore (mysis, brine, frozen, pellets)
  • The Pajama Cardinalfish is a carnivore.
  • Offer frozen mysis and brine shrimp, finely chopped seafood, and small marine pellets, fed once or twice daily.
  • Being naturally nocturnal, some appreciate a feeding near dusk.
  • They can be timid eaters, so make sure boisterous tankmates do not take all the food first.
  • Target-feeding shy individuals helps them keep weight.
  • A varied meaty diet maintains their condition and gentle, hovering activity.

Health

Signs of good health, and the common problems
  • Pajama Cardinalfish are hardy and disease-resistant in stable water.
  • The main risks are stress from aggressive tankmates, underfeeding when outcompeted, and marine ich following poor quarantine or unstable parameters.
  • Quarantine new fish, keep nitrate low, and watch for thin bellies or hiding, which suggest a fish is not eating enough.
  • A peaceful tank with good cover and reliable feeding keeps these fish in good health.

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 Cardinalfish yet.

Top resources for more info

Independent creators and organisations we would send a friend to. We have no affiliation with any of them, and nothing here is sponsored. Links checked 2026-10-22.

YouTubers

HumbleFish & ReefSaltwater fish disease, quarantine and treatment (Humble.Fish forum)BRStvReef tank setup, fish health and quarantine series (run by a retailer)FishOfHex AquaticsBeginner saltwater guides, avoiding common mistakes and tank disasters

Sites

Keep going

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Same axes, same care guides: the honest way to choose between types.

See all Cardinalfish side by side →Compare with other animals

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Is your enclosure big enough?

These are the smallest sizes that give good conditions, not legal minimums. More room is always better.

Minimum 160 gallons, for a group of 3

Every figure, and where it comes from

For a group of three. Aquariadise asks at least 40 US gal for a small school, and LiveAquaria at least 30 US gal. Austria's law asks at least 130 gal for one or two cardinalfish and 20 percent more for each further fish, which is 160 gal for three, and this site sets its figure there, because good conditions are never less than a law requires. Under the same rule each fish beyond three adds another 26 gal.

Minimum volume160 gallons
CompanyCannot be kept alone. The minimum is 3, and the space above is for that group.
The recognised standards do not agree on this one
Austria, 2. Tierhaltungsverordnung, Anlage 5 (legal minimum): At least 130 gal of water for one or two cardinalfish, plus 20 percent for each further fish. A legal minimum. For three fish it comes to 160 gal, and the volume this site uses is set to match it. Checked 2026-10-23.
Aquariadise (pajama cardinal care guide): At least 40 US gal for a small school. Below the volume used here. Checked 2026-10-23.
LiveAquaria (retailer care card): At least 30 US gal for a small school. A pet-trade minimum, below the volume used here. Checked 2026-10-23.
Practical Fishkeeping (cardinalfish for the reef tank): A slightly more spacious tank for a group than the Banggai needs. No figure is given; the same article puts a Banggai pair at 26 gal. Checked 2026-10-23.

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.

My kit says 0.25 ppm. Is that dangerous?

A liquid kit measures total ammonia, but only the un-ionised fraction is really toxic, and that fraction is governed by pH and temperature. Each whole pH unit multiplies it roughly tenfold. So the same reading can be nearly harmless in one tank and lethal in another; this works out which one you are in.