Pajama cardinalfish mouthbrooder

Pajama cardinalfish

The Pajama Cardinalfish is a small saltwater aquarium fish that is nearly perfect for almost any tank. Visitors may wonder if the colors and patterns are real. Beginners will appreciate how easy it is to take care of them. They will get along with just about any other fish (that won’t try to eat them) and may even pair up and breed in your tank.

If you want to learn more about the very popular Pajama Cardinalfish, keep reading below!

Quick Facts About the Pajama Cardinalfish:

  • Scientific Name: Sphaeramia nematoptera
  • Max Size: About 3-inches 
  • Minimum Tank Size: 20-Gallons
  • Aggression Level: Peaceful
  • Care Level: Beginner
  • Lifespan: 2 to 5 Years

pajama cardinalfish in reef tank with corals

Natural habitat

The pajama cardinalfish is native to the western Pacific Ocean and parts of the Indian Oceans and can be found in the reefs around Australia, Fiji, and Japan. This species prefers to stick close to reefs where it can safely retreat to when predators are near. The Pajama fish are shoaling fish that develop a hierarchy within the group.

Proper aquarium conditions

As mentioned earlier, the Pajama cardinalfish is a great choice for nearly any saltwater tank. They are relatively small, hardy, and acclimate well to life in an aquarium. If you can keep your water parameters consistent, they are likely to thrive. Like most other saltwater fish from a tropical reef, they prefer the following conditions:

  • Temperature ~ 78 degrees F
  • Specific gravity ~ 1.026
  • pH ~ 8.0-8.2
  • Ammonia, nitrites, nitrates ~0 ppm

These are fairly standard for any tropical saltwater tank.

Since these are naturally peaceful shoaling fish, it is recommended that you keep 3 or 5 individuals to enable the most natural behaviors and the security/safety of a shoal. Given their relatively small size, individuals can be kept in tanks 20-gallons or more in volume.

Preferred location in the tank

From what I have observed in my tank (yes, I love these little guys and have them in my aquarium), they are a bit shy. Shy fish tend to do a bit better when there is plenty of live rock and coral structure–so they know where to escape to in the event of danger. Danger won’t happen that often, hopefully, in your tank at home, but they don’t know that.

In my tank, they tend to occupy the upper 1/2 of the tank volume, hovering/swimming between the surface and the green star polyps or polyps of my frogspawn colony. So they are a docile open water swimmer. They won’t zip around the tank like your tangs or angelfish, but they will nicely fill in that open water with a peaceful, quiet, constant presence.

Pajama cardinalfish is reef safe

So keep that in mind, as you plan out your community of reef fishes, as you try to create balance and interest at each zone of the tank.

Substrate preferences

Other than the need for structure/hiding places, you could keep them with just about any substrate scenario–bare bottom, coral rubble, light sand or even a deep sand bed would all be appropriate.

Water flow preferences

Assuming there is enough room in your tank to pick their preferred spot, they tend to find those pockets or relatively peaceful flow and just chill. That is, until feeding time.

pajama cardinalfish and caulerpa macroalgae

Time of day

One of the particularly interesting reasons to keep this fish is that they will provide interesting behaviors both during the day and at night time. At first glance at the fish, what you will find is a crazy/funky coloration pattern, and these big red eyes…the better to see you with…at night. That’s because Sphaeramia nematoptera is very active at night. If you wish to care for them in the most natural way, feeding at night would be recommended. More about feeding below.

Feeding the Pajama cardinalfish

Feeding time is the one time you will find these peaceful creatures are not-so-shy. In my experience, the Pajama cardinalfish I have had the opportunity to care for are not picky eaters–they will eat flakes, pellets, frozen Mysis, frozen brine shrimp, live blackworms–pretty much whatever you feed them. As with most fish species, live or frozen whole foods will likely be preferred over the processed alternatives. Some individuals may not accept commercial foods at first but can be acclimated to your preferred food over time.

Even though PJs have been spawned in captivity and are available through aquaculture facilities like ORA farms, the fish you find in the store may be wild-caught. Wild-caught fish are used to eating their preferred, natural live foods, so there are reports of some fishes struggling to acclimate to aquarium foods. If you do end up with a picky eater, the best bet to transition them to accept more foods is to introduce alternative foods at the same time and gradually decrease the amount of their preferred food.

Sphaeramia nematoptera lifespan is 2-5 years
Lifespan 2-5 years

Often that is all it takes. Once you’ve transitioned all the way to the preferred food as the predominant item in the food blend, try skipping a feeding (assuming they are otherwise fat and healthy) and then feeding only your preferred food for 2-3 days. Chances are, you can get them hooked by then. If not, revert back to the blend and try again.

They are considered to be reef safe and won’t nip at the fleshy polyps of your prized invertebrates. making them safe to have near reef life.

Evening eaters

These fish are most active during the evening hours. You will need to feed them at night rather than in the day to make sure they are eating enough. If you keep your tank cared for then, this species will even naturally start to breed. S. nematoptera is an excellent pick for beginners who have always wanted to raise fry to adulthood. All the larva of the Pajama fish require is to be a natural diet of rotifers and microalgae to survive. Keep in mind that this species is a unique mouthbrooder who houses there eggs inside of their mouths.

Is the Pajama cardinalfish reef safe?

The Pajama cardinalfish is definitely reef safe and will leave the fleshy polyps of your corals and the sometimes tempting (for other fish) mantles of clams alone.

PJ cardinalfish one of five saltwater starter fish
Image by Brian Gratwicke


The Pajama Cardinalfish is a peaceful species that will get along with most other saltwater fish. The small size of this species does make it a bad pick for most semi-aggressive fish. At a little over 3-inches, the S. nematoptera can be an easy meal for larger, predatory species.

Since they are timid and docile, by nature, you also want to avoid keeping them with aggressive fish, like the blue devil damselfish or anything likely to enjoy chasing timid fish.

Some recommended tank mates are Gobies, Clownfish, Royal Grammas, and other Cardinalfish.

As mentioned earlier, it is strongly recommended to keep Sphaeramia nematoptera in groups of 5 or more, if you have a big tank. But don’t let this be a deal-breaker, though. If you have ample hiding spaces and other docile species, they will also acclimate well.

small pajama cardinalfish

Reproduction & Sexing

It is difficult to tell male and female Pajamacardinalfish apart. To illustrate the point, here is a quote from The Complete Illustrated Breeder’s Guide to Marine Aquarium Fishes (a book I strongly recommend, despite the lack of help this next passage provides):

On sexual dimorphism

Sexual dimorphism is when there are externally detectable differences between males and females. In his authoritative guide, Matt wrote:

“Some reports suggest females are larger than males, but in my experience males are slightly larger than females…”(Wittenrich 2007). I certainly appreciate the attempt at clarity, but for those of us who haven’t witnessed spawning, fish size may not be helpful.

Later he describes some differences visible in mature, spawning adults–female abdomens swollen with eggs, or post-spawn males holding eggs in their mouths. But none of that is helpful in picking any two individuals at the store and hoping for a pair.

“Most specimens available at local aquarium stores are juveniles and will not display such dimorphism,” (Wittenrich 2007).

Establishing a bonded pair

Pairing up, therefore, in my estimation, is best achieved by allowing a group of fishes to sort it out on their own. With that said (or typed), however, since they are so peaceful, pairing up within a group is reasonably easily achieved.

Pajama cardinalfish

Spawning is even reportedly easy (although I have not personally achieved this…YET), but it is the raising of the fry that presents the greatest challenge (Wittenrich 2007).

Perhaps the most interesting aspect of cardinalfish behavior (including PJs bout not limited to PJs), is that they are mouthbrooders. During a spawning event, the female releases her eggs, which the male quickly fertilizes (that much is true in most fish species). However, after that, the male scoops up the eggs into his mouth, where he cares for the egg mass until hatching.

Many breeders have noted that it takes the males time to get the incubation period right. The first few times a male carries his eggs, he will most likely drop or swallow them out of fear or malnutrition. The Pajama fish will spawn every two weeks in the right conditions giving him plenty of attempts to master mouthbrooding.

Spawning most commonly occurs in the evening, when no one’s watching (can you blame them?).

Breeding pajama cardinalfish and raising the fry

Diving deeply into the subject of breeding pajama cardinalfish and raising the fry is a bit beyond the intended scope of this introductory article–and also beyond my current domain of personal experience–although I do hope to be able to report back to you, at some point, with some success.

If this is a topic of interest, I strongly recommend you pick up a copy of Matthew Wittenrich’s book, The Complete Illustrated Breeder’s Guide to Marine Aquarium Fishes (available on Amazon). The book covers the topic of breeding in great detail, is so much fun to read and plan out your own adventures.

Pajama cardinalfish mouthbrooder

Spawning quick facts:
  • Complexity: Easy to pair/spawn, low/med complexity to raise fry
  • Spawning cycle, once mature: About every 13-22 days
  • Size of clutch: thousands of eggs
  • Hatching: About 1 week at 80 degrees F (Wittenrich 2007)

Where To Buy 

You will have no problems finding the Pajama Cardinalfish. Most of the stores I’ve visited order them–including the big box store that carries saltwater fish. They are also relatively inexpensive, priced around $15 each (give or take).

Picking a healthy fish

If you are shopping at a local fish store and can pick your preferred fish, you want to look for the following:

  • Clear eyes (not cloudy or white)
  • Full fins (not torn or frayed)
  • Extended fins (not clamped down tight with the body)
  • Alert and out (not hiding/cowering unnecessarily)
  • Full body (not thin and malnourished)

You can see in this poor little guy below, the eye is cloudy and infected. The fin is just a little nipped.

sick saltwater fish with cloudy eyes, nipped fins

Whether to Buy a Pajama Cardinalfish or Not

If you’re looking for a peaceful, colorful species to add to your community reef tank, it is hard to beat the Pajama cardinalfish. Colorful, bold yet peaceful, hardy, tolerant, inexpensive, reef safe, and relatively long-lived, the only downside is how ubiquitous they are. These fish are suitable for beginners and experts.

In fact, they are one of the first aquarium fish I recommend you add to your tank.

Some other reading you may enjoy

There are a few interesting cardinalfish species that are perfect for the saltwater aquarium. You may want to consider keeping the Banggai cardinalfish, as well.

Learn more about the pajama cardinalfish

If you have a few more minutes, check out this short video to learn more.



Michael, Scott W. Marine Fishes: 500+ Essential-to-Know Aquarium Species. 2001. T.F.H Publications. Neptune City, NJ.

Paletta, Michael S. The New Marine Aquarium: Step-by-Step Setup & Stocking Guide. 2001. T.F.H. Publications. Neptune City, NJ.

Ulrich III, Albert B. The New Saltwater Aquarium Guide: How to Care for and Keep Marine Fish and Corals. 2014.

Wittenrich, Matthiew L. The Complete Illustrated Breeder’s Guide to Marine Aquarium Fishes. 2007. T.F.H Publications. Neptune City, NJ.





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