Pajama cardinalfish (Sphaeramia nematoptera) are relatively hardy and they are popular choices for beginners looking to keep tropical marine fish. Pajama cardinals are not naturally schooling fish, so they won't become distressed if they're kept alone. However, many aquarists prefer to keep community tanks, as they're more vibrant and fun to watch.
Meet the Pajama Cardinalfish
Pajama cardinalfish are sometimes known as polka-dot or spotted cardinalfish due to their interesting markings. They have yellow faces, dark bands that run from their first dorsal fins to their pectoral fins, and back halves that are silver with red dots. These nocturnal fish have large eyes that allow them to see better in low light. They tend to reach around 3 inches in length. To mimic their natural habitat, plant plenty of sea grass and provide rocky areas and overhangs for them to hide.
Living With Other Pajama Cardinalfish
While a pajama cardinalfish will be content living alone, you may want to keep a pair or two so you can observe their interesting mouthbrooding behaviors. While a single pair can live in a tank with a minimum capacity of 40 gallons, if you have two or more pairs, you need an aquarium of at least 75 gallons. Housing two or more males in an environment that's too small can cause them to become territorial and aggressive.
Other Potential Tank Mates
Since pajama cardinalfish are naturally friendly and peaceful creatures, you can keep them with any fish of comparable size who share the same water requirements. Good choices include other species of cardinalfish, gobies, clownfish, smaller species of angel fish, blennies and tangs. They're safe for reefs, so you can keep them in a tank with corals and sponges.
Tank Mates to Avoid
Don't house your pajama cardinalfish with any fish large enough to make the cardinalfish their dinner. As they're passive, they may get bullied by more aggressive species -- such as large hawkfish, dottybacks or hostile wrasses -- so ensure that you house them with other peaceful species. Although they're generally friendly, they're not 100 percent invertebrate-safe; avoid keeping them with small shrimp or other mobile invertebrates. Pajama cardinals may also pose a threat to much smaller fish, such as nano-gobies; it's best to keep them with similar-size tank mates.