What you are seeing in your fish, and in the link you posted is very interesting and cool, but there is nothing unusual about it. It simply shows the normal process of expression of secondary sex characters in Metynnis fasciatus (tiger dollars), which more or less follow a similar path as that in other Metynnis species. Among the Metynnis species known (~16 spp.), the most similar to M. fasciatus is probably M. hypsauchen, which is nearly identical sans the stripes. Together they seem to be part of one of the ~3-4 groups of species that comprise the genus Metynnis.
I was able to document the sexual development of my group of tiger dollars, as shown below in a few chosen images.
My group consists of 4 fish (I wish I had at least 6, and there there would be at least 2 or more females). Alas, only 4, and when they came (mid-2021) there were quite young and completely undifferentiated, so it was luck of the draw that determined that I ended up with 3 males and a single female.
General view of 75g tank they went in n Nov. 2021 -
Same date (Nov. 2021) - Genders undifferentiated
Still largely Undifferentiated (Dec. 2021). However, fish at left had began to develop a small lobe midway along outer edge of anal fin. Fish at right did not and that fish is my current female. Note that at this stage, red pigment is present on anal fin of BOTH genders.
By late January 2022, differentiation was happening... (male at left, female at right). Male fin has lobe, and is less red.
May-June 2022. Process continues, now nearly fully differentiated. Female above (more red, straight fin), male below.
By Dec. 2023, now in a 6-foot tank, they are fully differentiated and frequently showing horniness. Female in foreground.
Jan-March 2024 - Full adults. The female (at center) is slightly larger than the three males, but I don't know if that is always the case or just in my group.
same date (2024) portrait (male left, female right).
Cheers!
