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Is your tank cycled processing waste with the nitrification cycle?
More info below in my signature link for aquarium keeping, very helpful, and full of good info.
The black one looks fairly large across the lower area.
You are looking for the fish to be squared around the front and lower part of the body, tighter and somewhat shinier.
I just posted this answer / info re: birthing for Guppies, but these guys are also live bearers and the info is basically the same so should be helpful.