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Question
i am going to get a larger tank under $200. What size and what place should i get the tank that is cheap, like a setup kit. i have 24 black molly fry that are about 3 weeks old. When can you start to tell the differece between the sexes? when can they start breeding? why do they all look like females in the beginning? what happens to the fin (do they evolve into males) ?


Answer
A good place to find a decent tank cheap would be your local classifieds, ebay, or craig's list.  for mollies, I would suggest 30+ gallons or larger as they breed like crazy.  I breed guppies from time to time, and i find that i can usually tell the difference between males and females at around 4 weeks of age.  The females can show their gravid spot around then.  Males do not develop their specialized reproductive fins until around 2 months old.  I do not know exactly what happens to the fin  or why they all look like females from the beginning.