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
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. While these numbers provide a good guideline, stability is more important than hitting exact values. Pajama cardinalfish, like most marine species, are sensitive to sudden swings in temperature, pH, or salinity. Rapid changes can cause stress, lower immunity, and make them more susceptible to disease. Focus on maintaining stable, consistent parameters rather than chasing specific numbers.
Social grouping and behavior
Pajama cardinalfish are naturally peaceful shoaling fish, and keeping them in groups allows for their most natural behaviors and provides security. Ideally, keep them in odd-numbered groups of at least 3, with 5 or more being preferable in larger tanks. In groups, they display social interactions, establish a loose hierarchy, and are less likely to hide or become stressed.
Keeping a single Pajama cardinalfish or just a pair is not recommended. Singly kept individuals often become chronically stressed, hide frequently, and may have shortened lifespans. Pairs may not provide enough social stimulation and can sometimes result in one fish dominating or bullying the other.
As Pajama cardinalfish mature, some aggression or bullying can occur within groups, especially if the tank is too small or lacks adequate hiding places. This is usually related to the establishment of a social hierarchy. To minimize aggression:
- Provide a tank of at least 20 gallons for a small group, with larger tanks (30+ gallons) preferred for groups of 5 or more.
- Include plenty of live rock and coral structures to create hiding spots and break lines of sight.
- Monitor for signs of bullying, such as torn fins, persistent chasing, or a fish that is excluded from the group and hides constantly.
If you notice persistent aggression, consider rearranging the aquascape or, in severe cases, separating the bullied fish. In most cases, a well-structured tank and appropriate group size will allow for stable social dynamics and healthy, active fish.
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.
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.
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.
Pajama cardinalfish are naturally crepuscular to nocturnal feeders, meaning they are most active and willing to eat during the evening and nighttime hours. In captivity, if you only feed during the day, you may notice some fish seem disinterested or miss out on food, especially if they are shy or outcompeted by more aggressive tank mates. Feeding after the main lights go out, or under dim “moonlight” LEDs, can help align with their natural instincts and ensure all individuals get enough to eat. This is especially important if you are attempting to breed them, as proper nutrition and feeding timing can directly impact spawning success and fry survival.
Signs that a Pajama cardinalfish is not eating enough due to mistimed feedings include weight loss, persistent hiding, or lack of response at feeding time. To address this, try offering food shortly after lights out when the tank is quiet, using dim lighting if needed. Observe to ensure all fish are eating, and adjust your routine as necessary.
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.
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.
Pajama cardinalfish 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 and corals.
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 their 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.
Image by Brian Gratwicke
Compatibility
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.
Because Pajama cardinalfish are slow swimmers and naturally timid, they are vulnerable to aggression and bullying from more assertive or territorial tank mates. Their nocturnal habits and gentle nature mean they can be easily outcompeted for food by faster or more aggressive fish, especially during daytime feedings. Signs of incompatibility include persistent hiding, torn fins, weight loss, or refusal to feed. If you observe these behaviors, reassess your tank’s stocking and feeding routines.
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.
Reproduction & Sexing
It is difficult to tell male and female Pajama cardinalfish 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.
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 but 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.
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)
Challenges in raising fry
While spawning is relatively straightforward, raising Pajama cardinalfish fry is where most aquarists encounter difficulties. The main challenges include:
- Feeding requirements: Newly hatched fry are tiny and require live foods such as rotifers and microalgae. Without access to appropriately sized live food, most larvae will starve within a few days.
- Predation risk: In a community tank, adult fish or even the parents may eat the fry. Successful rearing almost always requires transferring the male (while brooding) or the newly released fry to a separate rearing tank.
- Specialized setup: A dedicated rearing tank with stable water parameters, gentle flow, and regular feeding of live foods is essential. Water quality must be carefully maintained, as fry are highly sensitive to ammonia and other toxins.
Most failures occur at the larval stage due to starvation, poor water quality, or predation. If you wish to attempt breeding, be prepared to culture live foods and set up a separate nursery tank for the fry.
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)
Clear eyes, full fins, and alert behavior are all indicators of a healthy fish. Cloudy eyes, frayed fins, or lethargy can be signs of disease, injury, or chronic stress. Selecting healthy individuals reduces the risk of introducing problems to your tank.
It is also important to quarantine any new fish before adding them to your display tank. Quarantine helps prevent the introduction of parasites or pathogens that could harm your existing livestock. A basic quarantine protocol involves keeping new arrivals in a separate tank for 2-4 weeks, observing for signs of illness, and treating as necessary before introducing them to your main aquarium.
You can see in this poor little guy below, the eye is cloudy and infected. The fin is just a little nipped.
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.
- Check out these other great beginner saltwater fish.
- If you’re new to the hobby, learn how to set up a saltwater aquarium.
References
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.