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I have kept tetras for a few years now and have noticed something that I find odd. Whenever I watch them they will be schooling in odd numbers. If I have 6 sometimes 5 will school together and sometimes they will break up the school to three and three but I have never seen them all schooling together or in groups of two.

Has anyone else noticed this? Why do you think this is?

I have also noticed this behavior. I have 8 neons and there is always one out, or a couple schools of 3 and 1 out.
same with my head and tail lights. I would like to know why this happens

-Vincent

First question: are the Tetras the dominant fishes in the aquarium?

If the Tetras don't have a large companion that looks threatening and predatory, such as a Blue Gourami, and they're the biggest fishes in the aquarium, then they'll be much less concerned about shoaling. Shoaling is a defence against predatory attack, and if the Tetras learn that they're the top dogs in an aquarium, then they'll start milling about in the manner you describe. If they have something that looks potentially threatening in the aquarium with them, even if the 'threat' fish is actually peaceful and unlikely to pose a real threat, they will school more tightly in response.

In fact, I'm willing to bet that if anyone tries an experiment along these lines, setting up two otherwise identical aquaria with identical Tetra shoals, keeping one as a control without a 'threat' occupant, and putting a more dominant fish in the other, the Tetras in the control will be fairly nonchalant about shoaling, while the ones sharing their home with a bigger fish such as a Blue Gourami will display much more obvious tight shoaling behaviour. It's even possible that the Tetras will recognise certain other species as a potential threat in the wild, and react more strongly to, say, the presence of a Cichlid fish from their native waters than to a Gourami, though again, you'd need to conduct experiments with several identical aquarium setups to confirm this, and spend a LOT of time observing them.

If your Tetras are the dominant fishes in the aquarium, you'll also notice that the males in particular (especially in those species that exhibit noticeable gender differences such as elongated finnage or colour differences) will start to exhibit the behaviour I've labelled "part time territoriality", where they adopt 'landmarks' in the aquarium as places from which to display to each other and to the females. The most extreme example I've noticed is Beckford's Pencil Fishes - if they're the dominant fishes in the aquarium, then in my experience you're in for fireworks, because mine started behaving more like Melanochromis chipokae Cichlids than Characins!

I've noticed considerable levels of this behaviour in Lemon Tetras too, though their 'jousting' activities are all show and bluster - they 'pull their punches' when the males make flying passes at each other. Rosies, Black Phantoms and similar species can be expected to behave likewise. Beckford's Pencils, on the other hand, can turn nasty. In the case of that species, my recommendations are [1] give them a good deal more space than their small size would otherwise suggest, to allow their territories to be spaced apart [2] keep more females than males [3] put something in the aquarium that will be a 'dominant' fish and encourage them to shoal more.

Then of course, some species are naturally tighter shoalers than others under identical circumstances. Red Eye Tetras are pretty tight shoalers, while Lemons (particularly in an aquarium where they don't have a 'threat' fish to contend with) will break into loose assemblages in the manner you describe, the males jousting with each other to see who's top dog when it's time to mate. Another fish that exhibits the territorial behaviour very well, though without the nastiness of Beckford's Pencils, is the Silver Tip Tetra. You'll notice that in all these instances, the species that exhibit the most marked territorial displays are those that either [1] have noticeable differences between male and female individuals, such as the elongated dorsal and anal fins of Black Phantoms, or [2] have high contrast markings on their unpaired fins, such as Lemon Tetras and Silver Tips. Species which have clear fins such as Cardinals are much less likely to exhibit this kind of behaviour.


Panda Catfish fan and keeper/breeder since Christmas 2002

the thing is that mine have predatory fish such as gourami's in it. Also, large plecos might threaten them.

-Vincent

Hmm, interesting.

I've noticed that Neons and Cardinals are somehat lackadaisical shoalers in any case, but Head And Tail Lights - this surprises me. I'm accustomed to seeing them being fairly good shoalers.

However, fish are individual like you and I - even supposedly 'identikit' shoalers like Characins. My cave dwelling Cardinals have taught me that!


Panda Catfish fan and keeper/breeder since Christmas 2002

I think that I will do that experiment. I've decided to have a large shoal of tetras in my 55 gallon tank but I will need to quarantine them. I'll just keep half of them in one quarantine tank and half in another with a "threatening" fish. I'll see if other fish will influence their preference to odd numbered shoaling.

If you want a really unusual example of Tetra behavior, there have been a couple of reports (one, I think, from Axelrod himself) of seeing a single Cardinal Tetra in the wild apparently guarding a large school of fry.