Yellowtail Blue Damselfish

Just one of these in the tank.  There used to be two... the second was the nastiest fish I've ever seen.  Hid in the rocks -- extremely territorial.

Scientific Name : Chrysiptera parasema
Common Names : Blue Yellowtail Damselfish, Goldtail Demoiselle
Yellowtailed Blue Damsel Care Level : Easy, good for saltwater beginners, but don't use them to cycle your tank. This is not needed with the use of live rock as the primary biological filter.
Life span : 4 - 6 years, possibly longer
Size : Up to 3 inches (8 cm)
pH : 8.1 - 8.4
Temperature : 75°F - 80°F (24°C - 27°C)
Specific Gravity : 1.020 - 1.025
Carbonate Hardness (dKH) : 8 - 12°
Origin / Habitat : Indo-Pacific
Temperament / Behavior : Much like other damsels, this one can be moderately aggressive. Aggression can be lessened or spread out if you have 3 to 7 in your aquarium.
Breeding / Mating / Reproduction : 
Tank Size : 30 gallon minimum
Compatible Tank Mates : Avoid keeping them in an aquarium with Lionfish, Snappers, Groupers, Triggers, Eels or any other predatory fish large enough to eat them.
Reef Tank Compatible? : They shouldn't bother the corals and/or anemones but can sometimes be little jerks to the other fish in a reef aquarium setup.
Fish Disease : Saltwater Fish Disease - Diagnose, Symptoms and Treatment
Diet / Fish Food : Omnivore - try to give them a variety of foods. They will take vitamin enriched flake foods, frozen and definitely live foods.
Tank Region : Middle to bottom
Gender : Difficult to determine