Cirrhilabrus, the “Fairy Wrasses”, are one of the most elegant, active, and colorful reef fish. Their appeal in a reef tank is common to many, but not all have a well-rounded understanding of the compatibility amongst them. Enter the notion of “complexes”: groupings of very closely related species within a genus. Complexes create groups in which the species are genetically very closely related. Physically the species in these groupings have similar structure: the body shape, fin shapes/sizes, and maximum length. Coloration is not necessarily a commonality within a group, but patterns are to a moderate degree. From a genetic viewpoint, it is quite likely each complex originated from a common ancestor or perhaps from one species still within the complex. Complexes have large implications on levels of aggression and compatibility amongst other complexes. For the most part, the level of aggression within a complex is similar for all those species it contains (of course, there are always exceptions). The chart below presents complexes within the Cirrhilabrus genus. The chart can stand on its own to guide a hobbyist in selecting various species for mixing, as it is based on the phylogram of the genetic heritage of the genus. It includes notations on aggression (and exceptions), minimum tank size, and sub-groups within each complex. The complexes are named simply by the most genetically senior species of the complex. The relationships between the complexes are represented are represented by the solid lines in accords with the phylogram. Within most complexes, there are also sub-groups. These sub-groups are worth distinguishing from the rest of the complex as their relative levels of aggression typically differ. On that note, the color shading of each box represents the general level of aggression relative to the others. Red boxes have the highest levels of aggression, while green have the lowest. A handful of species go against the norm of their respective sub-group, and those are designated by a “+” for those more aggressive and a “-” for those less aggressive. When seeking to mix two or more species within the same tank, the chart should be used in the following manner. The further removed two complexes are the more likely the odds of compatibility. Additionally, the odds of success also improve for similar levels of relative aggression between two complexes; the higher the aggression levels the less likely this will be successful. Two species within the same sub-group should never be mixed. If the tank is very large (over 200 gallons), it may be possible to mix two species with the same complex but within different sub-groups. There is one notable exception: those two complexes in red (Filamentosus and Scottorum) should not be mixed with any other complexes or amongst themselves unless the system is extremely large, say over 400 gallons. For example, mixing an Exquisite, Lineatus, and Hooded is rather safe. However, mixing a Conde, Rubriventralis, and temminckii would be problematic at best. Hopefully with this knowledge, this chart can prevent needless frustration and incompatible additions the next time you find yourself in your local fish store searching for your next Fairy Wrasse addition. Happy Reefing! If you love wrasses check out the Wrasse Lovers Thread. For more information on all other types of reef wrasses check out the All About Reef Safe Wrasses in Aquaria Thread. Featured image courtesy of Eric Zard (zibba).