Pajama Cardinal - Sphaeramia nematoptera

In a 40-gallon tank, 4–6 Pajama Cardinalfish is a good range, but the key is group structure. They tend to sort themselves into a loose hierarchy, and aggression is usually concentrated on one weaker individual. To reduce bullying, avoid keeping just two (the dominant fish may harass the other constantly). Start with at least 4 added at the same time, provide branching rockwork and caves so subdominant fish can break line of sight, and avoid mixing them with more aggressive cardinals. If you see one fish being relentlessly chased, increasing the group size or rearranging rockwork to reset territories often reduces aggression.

Pajama Cardinals are naturally crepuscular and often shy under strong lighting, which makes them hide and pale out. This is normal in bright tanks. To improve daytime visibility and color: use moderate rather than intense lighting, add shaded areas with overhanging rockwork or macroalgae, and keep them in small groups so they feel more secure. Over time, as they acclimate and associate you with food, they’ll usually spend more time in open water and show stronger contrast in their yellow midsection and red polka-dot posterior.

Pajama Cardinals do best on a varied, protein-rich diet with added carotenoids to support color. Feed small portions 2–3 times daily. Combine high-quality marine pellets (small size, ~1 mm) with frozen foods such as enriched brine shrimp, mysis shrimp, and finely chopped krill. Include foods high in astaxanthin and other pigments (e.g., certain reef-specific pellets or “color” formulas). Avoid feeding just flake or just brine shrimp, as that often leads to dull colors and weaker condition. Consistent, varied feeding plus stable water quality will keep their yellow belly vivid and the red polka dots crisp.

Adult Pajama Cardinals are generally safe with larger shrimp such as skunk cleaner, fire, and peppermint shrimp, especially once those shrimp are established. However, they may pick off very small or freshly molted shrimp and will almost always eat tiny crustaceans like copepods, amphipods, and baby shrimp. In a mixed-reef, they are considered “reef safe” with corals, but you should avoid relying on very small ornamental shrimp species (or shrimp fry) in the same tank if you want to keep every last one. Provide plenty of rock and hiding places so shrimp can shelter during molts.

Sexing Pajama Cardinals is subtle. Males often have a slightly longer, more pointed second dorsal fin and may appear a bit slimmer as adults, while females tend to have a rounder belly when well-fed. The clearest sign is behavior: during spawning, the female lays an egg cluster that the male picks up and holds in his mouth. If you notice a fish with a visibly swollen, “full” jaw that refuses food but otherwise looks healthy and hangs in a sheltered area, it’s likely a male carrying eggs. During mouthbrooding (about 2–3 weeks), reduce harassment by tankmates, avoid major aquascape changes, and don’t force-feed him. If you intend to raise the fry, many aquarists move the pair or the brooding male to a separate, gentle-flow tank just before hatching and provide rotifers or very small live foods once the larvae are free-swimming.