Agressive Cardinal Tetras?

  1. G

    Giogina New Member Member

    Hi there,

    I just bought my first group of tetras - a school of 30±5 fish. Well, it was supposed to be a school, looks more like a bar fight to me. They're all in the same third of the aquarium, but constantly pecking at each other. It's not like one of them gets bullied in particular, an attacked fish may well attack another one the next second... There's plenty of hiding space (which they use when I bother them), but they don't go hide from the fighting. I don't see any injuries (yet), but I don't like them stressing each other out like that...
    Another problem is that a few of them have a small yellowish spot on their upper lip, some kinda bacterial infection I guess? Gave them a methylene blue bath earlier today, I hope that'll help. Maybe that's the cause for the agression? Then again, the stress from the fighting probably doesn't help with the infection...

    About the tank:
    A 60L, set up ca. 3 years ago, heavily planted. The only constant inhabitants are some snails and shrimp, since there's nobody to take care of the tank for a few months every year :V Normally I catch some small lifebearers from a nearby pond to keep in this tank, and bring them back when I go travel. So I thought, might as well use it as quarantine tank for my tetras (ultimately, they're going into my new 300L at home, where I can make my flatmates feed the fish XD). So, lifebearers out, tetras in, added some blackwater extract because cardinal tetras should like that, right? There's not a whole lot of filtration (lots of plant surface area though), so I've been doing 20% water changes every day, just in case. Lighting is off for now.

    What do you think is wrong with them? Maybe they fight it out who's boss in the new group? Can it be water quality problems? Or maybe a bad group size? I'd be happy about any ideas :)
     
  2. AquaticJ

    AquaticJ Well Known Member Member

    So there’s 30 Cardinals in the 60L? If so, you’re beyond overstocked, which would explain the aggression.
     
  3. OP
    OP
    G

    Giogina New Member Member

    Hm you might be right - but then, they were not agressive at the store where they had like 200 of them in a 30L. They're young, about 1.5 cm long. And they don't even use the entire space they have :V Hm, I could try and put half of them into the other quarantine tank. However, there's freshly bought Corys in there, one of whom looks sick, he's breathing too quickly - so that might well go the other way, too :/
     
  4. OP
    OP
    G

    Giogina New Member Member

    I finally found the right thing to google - turns out, cardinal tetras only school when scared, and otherwise behave rather territorially. So when they move into a new tank, they start establishing a pecking order and who gets which spot. (If I had known that, I wouldn't have gotten 30 at once...)

    Can anyone here confirm that? I've seen a bunch of display tanks where they seemed to school most of the time, but usually those tanks have an even higher tetra density than mine currently does... But it's just as possible that me standing in front of the tank scared them and made them school, so I wonder what other peoples experiences are.

    By now, it's a bit more peaceful in my tank - they're more distributed, the only ongoing fight is between two of the larger ones who want to occupy the front bottom corner... So I think, for now, I'll prioritize not transmitting diseases between the tanks. They'll go into the big tank soon enough :)
     
  5. AquaticJ

    AquaticJ Well Known Member Member

    They’re very much schooling fish, and at my store we don’t see any particular aggression from them. In theory, the more you add to a school, the less aggression you should see. That goes for any fish, which is why you also see African Cichlid tanks filled heavily with them. However, if the tank is too small, it won’t matter how many you have.
     
  6. OP
    OP
    G

    Giogina New Member Member

    Interesting! Maybe it also depends on how much vegetation/hiding spots there are in the tank (no use fighting over a spot if everything is basically open water)? How much vegetation do you have?
    Looking forward to see how they behave in the big tank.

    How worried should I be about the yellowish spots that some of them have on their upper lip? Google says it might be columnaris, but so far they don't make the impression of having a deadly disease... Colorful, active, eating well. Should I keep doing methylene blue baths, or observe for now?
    (Sorry, can't really make a photo, my camera isn't good enough... It's beige and more like lipstick than any kind of growth.)