The two to one ratio is also very important for after the moment the female has the fry. After she had the babies, she is free to mate again, and if the females are in disadvantage in terms of numbers, males tend to aggress them. Females then get very tired, trying to escape all that male attention and even die.
I hope it helps.
You can also do an exchange at petsmart if you see anything wrong going on, like males pinching females
