Too many babies

Question
When we set up our tank we put 2 orange Platys,2 Black Mollys,and 2 neons. After about 2 months the Mollys had a single baby and then one of the parents(not sure which one)died.2 weeks later the Platys had a group of 3 babies,10 days later 4 more,10 days later 3 more,now the Black Mollys have had 4 babies together,we have baby fish in every stage you can imagine. Why is this happening are they supposed to have so many?Are the Mollys supposed to mate with their own offspring?Any answers you can provide would be greatly appreciated.Also how do we tell the males from the females.

Answer
Hello

Livebearers do have a lot of babies regularly. They can have 50 or more when large enough, but in a community tank many will typically get eaten leaving the fastest and sneakiest fry.

Livebearers often end up breeding with their offspring. When there is a lot of inbreeding through a few generations you will start to see genetic defects, usually bent or crooked spines. It is best practice to control inbreeding but typically you will not see defects unless you let it gor for a number of years. You could try selling off all your males once a year and getting a few new ones but there is a good chance they will be from the same genetic pool anyway!

If you look at the males anal fin it will be long and thin compared to the females triangular fin. In livebearers they call this male fin a gonopodium.