There pretty rare. Heres some quotes from a reef blog.
"While at LiveAquaria I had the chance to observe and photograph one of the first Rabaulichthys Anthias to make its way to the U.S., via wholesaler Russo’s Reef. This genus is commonly referred to as Sailfin Anthias for their large dorsal fin, as seen in the photo below of Rabaulichthys altipinnis by Dr. Jack Randall."
"The exact species, of the two specimens, that LiveAquaria acquired are unknown. It shares many characteristics with R. altipinnis and R. suzukii. Interestingly, Randall and Pyle came across 40 Rabaulichthys specimens that were dredged up from 40m at the Caroline Islands. No meristic differences from R. altipinnis were found, with the only physical difference being a more slender body shape. Given the poor condition of the dredged fish and the lack of meristical differences, Randall and Pyle decided to wait for additional specimens and observations before potentially describing a new species."
"The Rabaulichthys sp. that went to LiveAquaria does exhibit a body that is more slender than R. altipinnis–however they were collected in the Coral Sea, a few thousand km south of the Caroline Islands where Randall and Pyle found this undescribed “morph”. Additionally R. suzukii from Japan and R. stigmaticus from the Maldives also feature a slender body shape, but they differ from this specimen and R. altipinnis in coloration.
Rabaulichthys can be particularly finicky and easy to scare fish. This specimen was extremely difficult to photograph–had it not been for a tight fitting lid it would have jumped out of the aquarium on a number of occasions. Unfortunately we never did see it raise it’s dorsal fin–but check out this photo from the collection outfit Cairns Marine (Thanks Joe): Male Rabaulichthys sp. Displaying Dorsal.
The male’s stunning dorsal display is paired with rapid swimming motions, as used during courtship much like Cirrhilabrus Wrasses. Perhaps the additional female LiveAquaria has will be able to illicit the male to do a little showing off…"