Dominant Damsels

Ways to deal with dominant and aggressive damselfish in aquariums.

Q. I have a question regarding a far too dominant domino damsel (Dascyllus trimaculatus). I am an 11-year-old hobbyist, and currently run a wet/dry filter on a 75-gallon saltwater tank that contains two humbug damsels (Dascyllus aruanus), two blue devils (Chrysiptera cyanea), a beaugregory (Stegastes leucostictus), a yellowbelly damsel, a golden-brown dottyback (Pseudochromis fuscus), a domino damsel and a small angelfish.

My question has to do with the domino damselfish in my tank. It is far too aggressive. I already had one die due to its aggressiveness. What can I do to stop this aggression? Any help would be appreciated.

A. Wow, that’s a tankful of mean fish! As mentioned in the answer to the previous letter, many damselfishes are territorial in their natural environment. In most cases a single damselfish will occupy and defend a specific area, and the border of its territory may butt up against those of other damselfish. A group of damselfish territories that are continuous with one another are referred to as a territorial mosaic.

The members of the genus Dascyllus display a different type of territoriality than most of their cousins. These fishes live in aggregations in heads of branching corals. Group members will form dominance hierarchies, with larger individuals occupying the spots at the top of the pecking order. Group size is limited by the size of the coral head in which the Dascyllus live. Once this coral commune becomes too full, group members will exclude entrance by members of their own species, as well as related forms.

In the aquarium, the Dascyllus often behave quite differently. Although these fish do not display excessive amounts of aggression in the wild, especially toward non-related species, they will do so in the closed confines of an aquarium. This is especially true for larger individuals.

There is really nothing you can do about your Dascyllus except to remove it from the tank. Sometimes you can curb aggression by rearranging the aquarium décor, but in the case of your damselfish this is likely to provide only a temporary solution. Most of the other fish in your tank can be equally as aggressive, so it may be that in time, a hierarchy will form among the tank residents and aggression will become less common. However, it will always be difficult to introduce other fish to the tank unless the new fish is extremely aggressive and is able to shake up the present pecking order.

If the Dascyllus continues to wreak havoc with your other fish, try banishing it to a quarantine tank for a few weeks. When you reintroduce it, it will probably find itself near the bottom of the dominance hierarchy, which may reduce its aggressiveness. The problem with this is that another bully will probably move in to take up where the Dascyllus left off!

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Fish · Lifestyle