Okay.. Don't trust people at chain fish stores... For you can't put male and female bettas together. I'm surprised they haven't fought.
You can put 5 females together in a ten gallon, minimum, as it creates a sorority, but you shouldn't mix them with guys; that often leads to death. My suggestion is to seperate them or divide the tank right away. The reason why your female isn't eating is probably because she's stressed or hurt from the other bettas. Look at her; does she have any physical wounds or shredded fins?
That's from a fight. Keep her in the small bowl. In an old 1 gallon jug, fill it with water and dechlorinator. Buy some aquarium salt like this: http://www.petsmart.com/product/index.jsp?productId=10850845
and mix 1 tsp of it in the jug of water thoroughly. Use this as her water for ten days, as it will help heal wounds.
Instead of flakes, use pellets, as betta's are more interested in them, and often will ignore flakes. This could also be why she's not eating. Change to pellets. However, she is probably lethargic if she's staying at the bottom... Just go out and get the materials, and also if you filled this out by copying plus pasting and/or posted pictures it would help...
I don't blame you, it's the people at the pet store that mislead you. This happens all the time, and I'm glad you found this site
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?