Can a Pajama Cardinalfish Be Kept Alone?

In the wild, pajama cardinalfish live in schools. However, unlike many schooling fish, they don't necessarily need to live in a group in a home aquarium. In fact, keeping more than a pair may cause problems. You have to understand their dynamics to best choose their arrangements.

  1. In the Wild

    • In the wild, pajama cardinalfish live in schools. However within their schools, these fish form hierarchies. The more dominant fish within the school "bully" fish lower in the pecking order. In the confines of the aquarium, this behavior becomes more stressful for lower-ranking fish. While most schooling aquarium fish get stressed when kept without enough conspecifics -- since in the wild they school for protection -- in this case the stress of bullying outweighs the benefits of sharing an aquarium with others of their own species.

    Single Fish

    • Unlike many schooling fish, a single pajama cardinalfish can live in an aquarium without problems. To help the cardinalfish feel safe, make sure the aquarium includes lots of hiding places. Oddly, a cardinalfish with lots of hiding places will spend more time out in the open, since it knows it can retreat if it needs to.

    Pairs

    • For most aquariums, a single male-female pair of pajama cardinalfish is the optimal arrangement. You can pick a pair by examining the fishes' vents. A vent is a cone or triangle-shaped opening on the bottom of the fish between the ventral and anal fins. In males, there are two tiny, cone-shaped protrusions and in females there is a single cone. This difference is most obvious after feeding, so try to visit your local fish store right after feeding time to have the best chance at picking a male/female pair.

    Groups

    • Normally, if you keep a group of cardinal fish in groups, a single dominant pair will pick on the rest of the fish. However, in large aquariums of at least 100 gallons, you can potentially keep a group of cardinal fish if you provide plenty of hiding places for the lower-ranking members of the school.

    When Breeding

    • The pajama cardinalfish is one of the few marine fish that readily breeds in the home aquarium. When breeding, you will see the male with a distended throat. The males of the species incubate their eggs and carry their young in their mouths. However, if the fish get startled, they may swallow the eggs. The best way to ensure that the young survive is to isolate the male in another aquarium or a net breeder when you see the distention. Move the male by scooping in a cup rather than a net to avoid stressing him.