What is ATS?


Acquired Territoriality Syndrome

Nature of the Beast

We like to believe that our fish are relatively predictable, especially with regard to compatibility. For the most part, fish are predictable in that certain combinations will almost always be impossible and some fish probably will not really care about what else is in the aquarium. For example, Guppies and Shovelnose Catfish are always going to be a bad mixture as the catfish will regard the Guppies as snacks. Similarly, the Common Plecostomus (a.k.a., algae-eater) grows to such a large size that most other fish commonly seen in community aquariums will never bother it. However, what happens when expectations fail? A case in point:
"Right now, I have a gold gourami in a 20 gallon. A while ago, I tried to keep white clouds. I didn't get far before I found them with stomachs torn open, and I know it wasn't my gourami because my gourami didn't do one thing to them even when I was watching them for 30 minutes. Last night, I went to this awesomefish store, and got a golden wonder killifish. I did all my research, and made sure water conditions were right for it. I acclimated it last night, and my gourami didn't like it much. She kept on curving her body and flexing her fins, while the killifish was too stressed out from the hour long drive back home, and the acclimation to defend itself. I turned off the light that night, and then when I went into my room to check on them, it was ok. This morning is when the bad thing happened. I found my gourami constantly charging into my killifish until it basically couldn't get off the gravel and died.

With all those deaths, I almost feel like it's impossible to succesfully keep new additions with my gourami. How can I be sucessful in keeping new additions with my gourami? I'd really ant to add some interesting things to that tank, but mny hopes have went down." [sic]


Sometimes fish that we expect to be kind turn out to be rather devilish. What is worse is when a fish coexists peacefully with its tankmates for years, only to kill them when the owner's back is turned. Unfortunately, scenarios like the above, in which fish that were either thought to have gentile dispositions or were only peaceful for a time become vicious monsters, are fairly common. I used to hear a complaint from my customers at least once a weak that one of their favorite fish suddenly became mean and killed the other fish in the tank. It seemed like everywhere I turned I was hearing stories of schizophrenic fish. What was the cause of all the carnage? In some cases, the carnage was due to a simple miscommunication with regard to what species are and are not compatible. For example, even though Pictus Catfish only grow to about four inches in length they can still consume small fish like most Tetras whole. This is not the type of scenario I am talking about. The problem that used to bug me was that some combinations were supposed to work, but did not. In most of these cases that did not match up to expected outcomes I have since been able to trace the problem to a single source: ATS. ATS stands for Acquired Territoriality Syndrome and describes a situation in which a fish has decided to claim the entire aquarium as its territory. Fish psychology is typically a fairly mundane, entirely tangential, concern for the average aquarist, but ATS is one instance in which the mindset and tendencies of a fish cannot be ignored.
How does ATS happen? In order for ATS to occur, two opportunities have to be afforded to the fish: 1) fish that suffer from ATS have natural territorial or hierarchical tendencies and 2) fish that suffer from ATS have to be permitted to think, on some level, that the entire tank should be their space. More succinctly, ATS-prone fish (those at risk for suffering ATS) have to have natural bullying tendencies somewhere in their instinctual programming and they have to be given a certain degree of entitlement in order for ATS to take effect. As an illustration, consider Silver Hatchetfish. Silver Hatchetfish are generally considered to be fairly peaceful, but it is also well-known that Silver Hatchetfish are shoaling fish, preferring to live in fairly large groups rather than as single individuals or pairs. This social structure has particular rules and hierarchies soon develop as a result of those rules. Most aquarists do not notice the social classes of a Hatchetfish shoal due to the similarity of the constituents, but careful observers will note that some Hatchetfish bully the others, taking food first or occasionally ramming the subordinate fish. As long as the shoal is fairly large the hierarchical tendencies of Hatchetfish are "buffered" by the sheer number of possible ways in which the problem fish may interact with its own species, but Hatchetfish have been known to become pugnacious if they do not have shoalmates (not to be confused with tankmates, which are not of the same species). However, pugnacity, by itself, is not enough to label the condition as ATS. In order for ATS to take effect, the fish has to feel comfortable enough with its environment to claim it--all of it. Despondently, most home aquariums are very small when compared to the environments afforded to fish in the wild and it is very easy for a fish to memorize almost every cubic inch of an aquarium if it has lived in it for a while. Once this familiarity is reached and if the fish has no appropriate place to vent its aggression (like on members of its own species that understand its social rules), then ATS is a very probable outcome.
Moreover, ATS may occur in any community aquarium if any of the below conditions are present:
  • Shoaling fish are not kept in shoals (at least 4-6 individuals, more being better)
  • The tank is small and relatively unchanging (that is, the fish know where to find everything and it does not take the fish more than a minute or two to traverse the aquarium)
  • Territorial fish are present and well-established (been in the tank for a long time)
  • Any fish has been relatively isolated or allowed to live without other fish in its territory (increasing its sense of entitlement)
Also, some species, due to their innate tendencies, seem to be more prone to ATS. These species include, but are not limited to, the following:
  • Gouramies, Bettas, Paradise Fish, Climbing Perches--all Anabantoids
  • Tiger Barbs
  • Zebra and Pearl Danios
  • Hatchetfish
  • Goldfish
  • Peacock Eels
  • Rainbow and Red-Tail Sharks
  • Headstanders
  • Serpae and Black Skirt Tetras

How to Avoid ATS

Is ATS inevitable? Thankfully, ATS is pretty easy to avoid if an aquarist is willing to take the steps to prevent it. On the other hand, curing ATS once it has occurred is often much less probable and some aquarists find themselves owning a species aquarium that was originally intended to be a community aquarium. Since ATS is hard to deal with if it ever occurs, it is much better for aquarists to learn how to prevent it and, fortunately, prevention is fairly simple. The best steps that can be taken to prevent ATS include the following (cross-reference with the aforementioned list of problematic conditions):
  • Keep shoaling fish in shoals. This sounds simple, but it is amazing how many aquarists will not buy their shoaling fish new shoalmates when the old ones die. Most shoaling fish should, at least, be kept in a group of six. If the tank is not large enough or the stocking capacity of the aquarium is too small to accommodate at least six individual fish then a shoaling species should not be kept.
  • Rearrange the aquarium frequently (once every 1-3 weeks). This is especially important in small tanks--changing the environment keeps the fish from getting too comfortable. This rearrangement should include not only the placement of large structures like rocks and driftwood but also seemingly minuscule things such as water flow. Specifically, it is nice to have a small circulation pump in every aquarium so that water flow patterns can be altered from time to time (adjustable filters and airstones are also effective). Also, adding new decorations every now and then, perhaps alternating with old ones, helps prevent ATS.
  • If in doubt, over-decorate. If fish cannot see each other, then they are not likely to act aggressively toward each other. In fact, some fish combinations that have been posited as impossible in the past have been proven possible if there are enough decorations/obstructions in the aquarium. In short, a bare tank is likely to be a dead tank but the over-decorated tank will never be too full for the fish.
  • Keep the aquarium stocked. If an aquarium that is meant to have many fish is allowed to sit unused save one or two fish, then those one or two fish will most likely suffer from ATS in that they will be alone for so long that they will think the entire tank belongs to them.
  • Lastly, if feasible, buy an aquarium that is a size larger than is thought necessary. Sometimes, especially if ATS actually does occur, the best solution is to keep the problem fish with other fish that are bigger than itself. If the tank is small, then this option will not be available to the aquarist. Additionally, a large tank affords the aquarist more stocking opportunities than smaller tanks.
But the question remains: What is an aquarist to do if ATS happens? As mentioned, ATS is not easily curable and, most of the time, the best cure is the same as the prevention. That is, if ATS shows up, an aquarist should try to implement the above preventive measures to dissuade the problem fish from its poor behavior. Nonetheless, ATS is hardly ever completely cured and the aquarist is often forced to either keep the fish by itself or to add fish that are more aggressive than the aquarist originally intended. Simply, prevention is the best policy.