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Hi there!
I have a lemon tetra and I noticed the funniest thing a few days ago.. his bottom fin has a black/white/yellow strip now! I didn't know if this is normal, or like a sign of becoming an adult? Hes healthy as can be, swimming around and all!
I'm very excited for it! He looks like a cutey now!

In Lemon Tetras, the colouration of the anal fin is a gender diagnostic.

The foremost rays of the anal fin become a really intense lemon-yellow, and look almost as if they have been spraypainted with acrylic paint in fine specimens! The external margin of the anal fin (the part furthest from the body) develops a black edge. Look closely at this edge and you will notice the following:

A female Lemon Tetra has a thin black line at this point, almost as if drawn with a draughtsman's precision pencil. A male Lemon Tetra has a thick black band, in alpha males (the ones that get to spawn) this black band takes up a third of the width of the entire fin. Really fine males also develop flag like dorsals with really intense high contrast colours that are used for signalling in 'jousting' contests to establish hierarchical ranking, but this isn't a reliable gender diagnostic as subordinate males will have dorsal fins that resemble those of females. Even subordinate males, however, have a thick black anal fin margin.

Before I lost my Lemons to chemicals coming in through my tap, my males were spectacular. I'll see if I can dig out a photo for you ...

Here you go, Lemon Tetras - note the difference in the thickness of the anal fin band in these two specimens ...


Attached Image:

Lemon Tetras - left=female, right=male

Panda Catfish fan and keeper/breeder since Christmas 2002

Yep, you've got a lemon that is maturing. Once it becomes a full adult, you can really notice a difference in the coloring versus how it used to look. As another example, here is one of my male lemons in his normal coloration:

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And when he gets to Heaven, to Saint Peter he will tell: "One more Marine reporting, Sir! I've served my time in Hell."