Q: I have a 75-gallon reef tank that contains a purple tang. I have a total of three fish and have been trying to add a few more to the tank (a sailfin tang, a copperband butterflyfish), but the purple tang keeps attacking them. It backs into them and hits them with its tail spine, nips them and chases them. Usually the other fish end up hiding, and unless I remove them, they do not feed and then die. Is there anything I can do to make this fish less aggressive?

The tank has about 60 pounds of Florida live rock, and there are plenty of hiding places. I have seen aquariums that held purple tangs, sailfin tangs and copperband butterflies before, so I know this combination is possible. Is my tang just a deviant? Thanks for your help!

A: Well, I and almost every other marine aquarist I know have had to deal with similar problems. It is a common predicament because many reef fish naturally defend limited resources, such as food, shelter and mates. In the wild, reef fish are not indiscriminately aggressive. They display antagonism toward members of their own species and other fishes that compete with them for these resources — they recognize these competitors by shape, color and behavior. When a fish is collected for the aquarium trade, it brings all this behavioral baggage with it into its new aquarium home.

Our task, as marine fishkeepers, is to manage or curb these aggressive tendencies. We do this by carefully selecting the types of fish we put together, by introducing the fish into the tank in a logical order and by making sure there are enough necessary resources (e.g., hiding places, food) to go around.

Let's look at the natural tendencies of the purple tang (Zebrasoma xanthurum) for a moment. This fish feeds on algae and often defends a particular area of the reef from members of its own species and other herbivores, which includes other members of the surgeonfish family. Although there is no information available on the territory size of the purple tang, a similar related species defends an area of approximately 430 square feet. That would be equal to an aquarium that is 43 feet long x 10 feet wide in surface area, which is obviously larger than your 75-gallon tank. Therefore, when you add any fish to your aquarium you are throwing it into the purple tang's territory! This may not be a problem if the new fish you are adding has a dissimilar diet, but direct competitors will not be tolerated.

The sailfin tang (Zebrasoma veliferum) is not an acceptable addition to your tank because it is an algae eater that is similar in shape to the purple tang. As I mentioned previously, reef fish behave more aggressively toward species with a body shape similar to their own, because fishes that share a similar body shape usually have comparable food habits.

This is probably why the purple tang is intolerant of the copperband butterflyfish, as well. Although not a spitting image of the tang, like the Zebrasoma, the copperband butterfly is a deep-bodied fish. Also, the copperband butterflyfish feeds by nipping at the substrate, like a herbivore. One of the characteristics that a territorial fish will cue in on to recognize a potential food competitor is how it feeds.

You mentioned that you have seen tanks with this combination of fish before, but you neglected to say how large these tanks were. I am sure they were not as large as a purple tang's natural territory, but I bet these aquariums were larger than a 75-gallon tank. The more space the subordinate fish have to get out of the aggressor's way, the better!

Also, the sailfin tang and the copperband butterflyfish were probably introduced before the purple tang. In the future you can reduce the likelihood of having these problems by adding the most aggressive fish after the less pugnacious species have settled into their new homes. Many reefkeepers get into this dilemma because they want to add a good algae eater, like a tang, to the tank as soon as the aquarium's ready for fish. They add the tang, it claims the whole tank as its territory, and then attacks any potential competitor introduced after it.

If I were you I would forget the sailfin tang because it is just too similar to the purple tang. If you are adding any other fish that may be potential victims of your purple tang's belligerent attitude, I would remove the purple tang before introducing these other fish. If you have an extra tank (besides the tank you will use to quarantine the new specimen you buy), you could move the purple tang to this aquarium, place the other fish in your reef tank (after they have been quarantined, of course) and then reintroduce the tang to its original home after a week or so.

Although I believe this method offers you the best chance of success, there are other things you can do that might help reduce aggression. One technique is to rearrange the aquarium decor, which in the case of a reef aquarium can be a real hassle. Reef fish use the topography of the reef to recognize their territories. Therefore, if you change the layout of the tank, you have, in effect, moved the fish to a different location on the reef!

Habituating the tang to new fish before you release them into the tank may potentially diffuse aggression as well. You can do this with smaller fish by placing them in a gallon jar and submerging it in the tank. Leave the fish in the jar for a few hours and see if the tangs interest in the new fish diminishes. If the tang is still trying to get at the new fish in their glass enclosure, poke some holes in the lid of the jar and place an airstone inside. Leave the jar in the tank for a day or more, until the tang seems to lose interest in its occupants. When this occurs, pour the fish out of the jar into the aquarium. Watch the tang for an hour or so and see how it responds to the new introduction. You can also use a glass or acrylic divider to partition off a part of the tank if you are adding a fish that will not comfortably fit in the jar.

One way to divert the attention of an aggressive resident fish away from a new introduction is to add a small mirror to the tank. Territorial reef fish are always most intolerant to the intrusion of members of their own species, so when it sees its own reflection in the mirror it will vigorously attack it and try to drive it away. This will help to channel some of the tang's antagonism away from the new fish while it adjusts to its new surroundings.

 

Turning off the aquarium lights, or feeding your tang before you release the new fish, can also help to curb aggression to some degree. By the way, to you skeptics out there, not only have I seen the feeding technique work in my aquariums, but it has been scientifically demonstrated by ethologists that fish are generally less aggressive on a full stomach!

 

In response to your final question about if your tang is a deviant, although you might occasionally run across an individual that is abnormally aggressive, your tang appears to be behaving like a "normal" purple tang. They tend to be the most aggressive species in the genus Zebrasoma.

An Update on the Banggai Cardinalfish

A while back, I shared some preliminary observations on an incredible fish called the Banggai cardinalfish (Pterapogon kauderni) that was beginning to show up in the trade. I would like to provide some further information for the aquarist thinking about acquiring a group of these wonderful fish.

It turns out that the Banggai cardinalfish is not as docile as I first thought. Although these fish may not display any aggression toward one another when first introduced into a tank, it now appears that after they have been in the aquarium for a while (in my case, several weeks), one specimen, or perhaps a pair of individuals, will start picking on the others.

If they are being kept in a larger tank, individuals will disperse throughout the aquarium, which will decrease the number of aggressive encounters. But in small and even medium-size tanks (75 gallons or smaller) the dominant fish will persistently chase and nip subordinate individuals.

I ended up having to separate several of my specimens after one individual had its fins chewed up by an aggressive tankmate. It was also brought to my attention that the Dallas World Aquarium, which has a large group of these fish on display, has already had males carrying eggs.

Acknowledgments: Thanks to James Lawrence, Dennis Reynolds, Tony Wagner and Joyce Wilkerson for sharing their observations on the Banggai cardinalfish.