Damsels get a bad rap but in my experience this is due to some fundamental misunderstandings of the entire group. First there are many different groups which include damsels, clownfish, and Chromis. Each group has some aggressive members and others that aren’t so bad as anyone who has had a murderous maroon clown can attest. Next despite their small size they frequently do not make good fish for small aquariums. There are a few exceptions namely Ocellaris /Percula clowns and some of the smaller Chromis species. With the exception of the Chromis group nearly all other species form territories that they will vigorously defend, the size of these territories and how aggressive they are in defending it largely determines how much trouble they are going to cause you. For tank size the main consideration isn’t total water volume but how much territory is available for the fish to claim, so a tall skinny aquarium will have much less usable space than a shallow deep one.
The most suitable species defend very small territories (think less than 8”x8”) and in general are easily bullied by other fish that are even marginally bigger than them. This group would include
Chrysiptera hemicyanea, C.parasema, C. springeri, C. talboti, and the gorgeous
C. traceyi. For this group I would recommend a minimum tank size of approx 40 gallons if you wish to keep them with other small fish though any would be perfectly happy by themselves or with other feisty fish in a tank as small as 20 gallons. In a large aquarium (180g+) a group will each establish individual territories that they vigorously defend from one another and in general ignore anything else bigger than they are in favor of squabbling with each other. Also worth mentioning is that in my experience all damsels in this group will eat flatworms to one degree or another and are useful in their control.
This group of Azure damsels in my tank would swim up in the water column normally but at the first sign of danger flee back into the rocks. They were on the bottom of the pecking order this tank.
They next group as far as aggression would include
C. starcki, C.cyanea, C. taupou, and the
Pomacentrus sp. This group of fish is still not holy terrors that their cousins in
Dascyllus, Neoglyphyidodon, and
Ambylglyhidodon are but they can hold their own with more aggressive fish as well. Their territories are roughly double the size of the previous group and I would recommend a tank at least 75g in size for a single fish in this group. Once again in a large tank they will disperse and form territories that they fight each other for, coming together to breed when they feel the need.
The final general grouping includes nearly all species in
Dascyllus, Neoglyphyidodon, and
Ambylglyhidodon. These fish get large and can hold their own with triggerfish and other aggressive fish. They excel in being used as dither fish in aggressive tanks and many get large enough to not be immediate prey to predatory fish if given a head start. Sadly they are also sold to cycle tanks and as a result are frequently bought by beginners where they establish a territory that includes the entire tank and then murder everything else that is added after them. As a special not several species of
Neoglyphyidodon are sold as honey Chromis as juveniles, while pretty when young they grow up to be big mean grey/brown fish.
Appropriate tankmates for this group.
So some final rules, don't add them to a tank first, get the other fish in that you want that are not as assertive, make sure that you are buying from a group that is suitable to your needs, and finally don't be afraid of this group of fish, there are many that if they weren't so inexpensive would be highly sought out for their colors and patterns.
