Originally Posted by Otterfun
It's just that when I had the 2 baby cups side by side, one of them is showing vertical bars and got very active in front of the other one when I first got them. I thought one of them is being aggressive versus stressed or ready to breed....
The stripes confused me at first too. Sometimes I would look at my baby betta and it would have horizontal stripes (seen in young bettas and in stressed, adult bettas). Minutes later I would look and she would have no stripes, and then vertical bars/stripes started to appear as she got a little older. These patterns can change within seconds as I've seen it happen literally before my eyes. The best way to tell if they're stressed is to see if they have horizontal stripes and their color will become pale and less vibrant.
Unstressed baby betta with horizontal stripes:
Vertical "breeding" stripes (they develop as they mature and sometimes appear randomly):
The reason your baby got active and developed the vertical stripes was because she may have been displaying her readiness to breed, or to let the other fish know she is female. I'm not exactly sure on betta's breeding behaviors but I witnessed something similar to you once when I put my female betta in a plastic cup and floated her in my male's tank:
At the beginning of the video she has horizontal stripes and looks very pale, indicating stress. But after she notices the male betta dancing around her cup she develops vertical stripes (0:37) and seems very active and alert to where he is.
Sorry this was so long of a post, hope it helps.