New Member
Aug 5, 2020
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West Coast, Canada
Hello all,

I have a few questions about head-and-tail-light tetra behavior. I used to have 3 of these tetras in my planted 10 gallon glass aquarium with intentions to get more once I transfer them into my 30 gallon that I have not yet started to cycle... there are only 2 left- the two bigger ones. This brings me to my first question:

1. Did my tetras single the smaller one out because he was a runt, or because he was the smaller and weaker male?

I do not know their sex, this is an assumption and might as well be my next question:

2. How do you tell the sex of these tetra species?

The reason I assume it was the smaller weaker male is because the two that are left have started to circle around each other as if they are playing- or attempting to breed, every now and then they will dart at one another while they are circling together. This brings me to my last and possibly more concerning question:

3. Are my two last tetras attempting to breed?

Or is this something else...

Thank you in advance,



Tank of the Month!
Jul 28, 2009
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Theyre a shoaling species that ideally would be in a group of 10 or more. There could be a number of reasons why you already lost the smaller one.. if it was disease, this could be due to water parameters and not necessarily just 'dirty' water, but too high GH and PH.
It could have been attacked by the other two and generally outcompeted for food.
Females are generally a lot chunkier than the male and the visible swim bladder is rounded whereas males tapers to a point.
I doubt the remaining two are mating, they are likely quite stressed and taking it out on each other.
If you up their numbers significantly you should see some improvement.



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