Sphaeramia nematoptera

Sphaeramia nematoptera, commonly known as the Pajama cardinalfish, is probably one of the best known apogonidae. With their yellow head, black vertical bar and red dots on the back half of the body, it can't be confused with any other fish. It's very popular, probably due to the fact that it is friendly, reefsafe, easy and stays relatively small, so it can be added to most tanks.

They're best kept as a couple unless you have a very large tank, in that case you can add a group.