Aquatics Unlimited: Articles: Kings of the Castle
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THE COMMUNITY AQUARIUM: PRINCIPLE NUMBER 3

"Kings of the Castle"

The villains in our first two installments weren't really such bad guys; the gulpers weren't really mean-spirited, just hungry, and even the fin-nippers were more or less just out for a good time. But there are fish who seem to act with malice aforethought, who have both the ability and the intention to cause bodily harm to their fellow fish.

Picture this typical scenario: Reggie, your favorite angelfish, is cruising silently and ever-so-gracefully around your 55 gallon show tank, when suddenly he goes one step - er, stroke - too far and out of a pile of rocks comes Jack, your Jack Dempsey (you never did have much in the way of imagination, did you?) who promptly pounces on Reggie, ripping off a chunk of fin and sending scales flying. Reggie heads for that nearby pile of rocks, seeking it's protection, but that only seems to further upset Jack, who wallops him yet again. Finally, Reggie, mustering up what little strength he still possesses, limps (ok, it's not like he's favoring a leg, but you get the idea) slowly back to the other end of the tank. As Reggie gets farther and farther from that rockpile, Jack's vicious attacks lose their intensity and finally cease.

Reggie learned an important lesson - and so, hopefully, did you: like most cichlids, Jack is territorial. He sets up a place to call home, re-decorates it to his liking, memorizes a few boundary markers, and then defends it aggressively against any strangers who might come moseying by.

Dealing with territorial fish in a community tank can be one of the most challenging endeavors for the hobbyist, since the reality of the situation is that these fish just plain don't want to have neighbors, much less room-mates. But there are a few things you can do to help increase the likelihood of your success.

Consider the size, aggression and speed of the potential tankmates before you add them. The basic idea is that since each fish is treated like an unwelcome intruder, it had better be equipped to handle itself. If it's not as aggressive, it had better be bigger or faster, or well armored - or it had better be prepared to spend the remainder of its miserable life hanging out at the top of the tank hoping nobody notices him. A common example would be attempting to mix oscars, which are somewhat aggressive, with Jack Dempseys or firemouths which are quite a bit more assertive. If the oscars are smaller or even the same size as the other fish, they are fortunate to survive the rigors of battle for even a few hours; but if they are considerable larger, an uneasy peace will likely occur.

Choose fish that are as un-alike as feasible. The more an intruder appears to resemble the territorial fish, the more viciously it will be attacked. This is why male Siamese fighting fish go bonkers when being approached by other male bettas, push unwilling females away less vigorously, but pretty much ignore all unrelated fish. The purpose of territorialism is to set aside a space for that fish to do his own little thing. A fish, for example a plecostomus added to a convict cichlid, that does some other little thing might not be considered a threat and therefore tolerated, while a closely related or similar looking fish would be driven away. This rule, of course, does not overshadow the previous rule, so no, don't try your prize-winning delta-tail guppies with that mated pair of zebra cichlids.

Add the more aggressive fish later. There are certain natural advantages to playing the game in your home park: you know the turf and you haven't just suffered the rigors of travel. Thus, it's possible for a weaker fish to hold its own against a stronger one if the stronger one is the new kid on the block. A cichlid community might start out with oscars, then firemouths, then dempseys and so on and have few casualties - while reversing that order might very well end up a disaster.

Keep larger groups of fish. There is some safety in numbers. If the most aggressive fish has only one fish to harass, he will soon run that fish ragged; but if there are dozens to chase away, he most likely won't chase any single one long enough to endanger it. The mbuna African cichlids make good examples of this point. Put six in a tank and you will likely end up with one when they mature; put start with a dozen or more and they will likely all co-exist without any casualties.

Use larger tanks. Territorial fish don't want the world - just their own private little piece of it. If your tank is large enough that each fish or pair can set up their own separate miniature kingdom, they will more likely solve border disputes by false charges (as in pretend attacks, not overbilling their Master Cards) and stare-downs than by actual violence. Unfortunately, some fish define their personal space in square yards rather than square inches, so a "larger tank" might have to be huge to eliminate carnage.

Decorate heavily - or not at all. Territorial fish use your decorations as their lot lines, so they can tell whether or not somebody really is stepping on their lawn without waiting for the zoning commission to convene. So sometimes providing an abundance of markers allows for an abundance of separate little lots. Oddly enough, the opposite approach often works as well. If there are no decorations at all, the fish seem genuinely confused as to where their property begins and ends, and efforts to protect their homesteads become half-hearted.

Re-decorate when adding fish. (No, don't wallpaper the foyer, move the rocks and plants in your aquarium around.) The principle here is much the same as above. If you have, say, a dozen fish, each with it's own little territory, any new fish will have a hard time finding a place that isn't already staked out. Since none of the current residents is willing to subdivide, the newcomers usually get chased from one lot to the next until they find a really cozy spot - like wedged between the heater and back glass - to call home. A massive change of decorations erases all the lot lines, and as all the older fish struggle to set up new boundaries, there is a good opportunity for the new fish to set up housekeeping.

The cichlids are the most common group of fish to exhibit territorial aggression in the aquarium, though the degree of hostility varies from the relatively harmless angelfish and discus to the downright intolerant Red Devils, black belts and green terrors (I guess their names pretty much say it all). Many of the labyrinth fishes are also territorial, although most, like the blue/opaline/gold/platinum gourami are either too slow or uncoordinated to be much of a threat to many fish. Others, like the red snakehead, are among those fish voted "most likely to end up alone."

  • angelfish = Pterophyllum scalare
  • Jack Dempsey = Cichlasoma biocellatum
  • oscars = Astronotus ocellatus
  • firemouth = Cichlasoma meeki
  • Siamese fighting fish = Betta splendens
  • convict = Cichlasoma nigrofasciatum
  • plecostomus = Hypostomus plecostomus
  • guppy = Poecilia reticulata
  • zebra cichlid = Tilapia mariae
  • Red Devils = Cichlasoma labiatum
  • black belts = Cichlasoma maculicauda
  • green terrors = Aequidens rivulatus
  • blue/opaline/gold/platinum gourami = Trichogaster trichopterus
  • red snakehead = Channa micropeltes

This article originally appeared in
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Freshwater and Marine Aquarium Magazine

Copyright © 1997 James M. Kostich
All rights reserved.

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