Cardinal fishes are in many respects the damsel fishes of the night. Members of the family Apogonidae, they fill many of the ecological niches that are filled by the damsels during daylight on the reef. The night time has caused them to be less territorial (since there is less competition) and they have larger mouths ( to consume the larger plankton associated with night time on the reef) and posses large eyes. They do well in small groups and are non aggressive. They eat a variety of food and will adapt readily to captivity if they are not harassed by other fishes. They like subdued lighting and are a good addition to a "twilight aquarium". 

There over 170 species but only a few are seen in the hobby. Sphaeramia nematoptera is the common Pajama cardinal. It is tempting to add these fish to community aquarium , however they need a tranquil environment that is associate with either a reef or twilight aquarium. In captivity, if well cared for, Cardinal fish will breed. The male will hold the eggs in his mouth. On the reef they may be commensal ( to hang around with no "cost" to either host or visitor) with sea urchins or anemones and many other species are red to yellow in coloration with striking patterns.
 

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