Blue Damselfish(Chrysiptera cyanea)
The Blue Damselfish is probably the best selling marine fish in the United States. Beginning hobbyists relish its hardiness and small size, while advanced aquarists praise the color and activity this member of the Pomacentridae family brings to the aquarium. Female Blue Damselfish are completely blue. Males, on the other hand, have an orange tail and are commonly called the Orangetail Blue Damselfish or Blue Devil Damselfish.
Native to reefs across the Indo-Pacific, Chrysiptera cyaneais usually busy defending a small territory. Interestingly, the Blue Damselfish has the ability to hide in a hole or crevice and darken to an almost black color. This usually happens when it is threatened. After the perceived threat is gone, the Blue Damselfish will return to its electric blue color in a matter of seconds.
The Blue Damselfish is somewhat aggressive, so its housing should be large enough to easily accommodate multiple specimens. It is a good fish for beginners and makes an ideal companion fish for saltwater aquariums of over 30 gallons. The Blue Damselfish is also a great choice for reef aquariums with invertebrates. As the Blue Damselfish matures, it may demonstrate pronounced territorial behavior towards future additions to the aquarium. If keeping the Blue Damselfish with other damselfish, provide multiple hiding places to break up territories and decrease aggression
Info from http://www.liveaquaria.com/
Pic from http://www.livetropicalfish.net/