Some fish, usually live barers which are swordtails, platies, and guppys can infact change sex. But only female to male. If the female is not getting enough attention she will convert into a male. With guppies you tell by that little fin on the bottom of them, if it's all bunched up and straight it is a male, if it's fanned out it's a female. I actully just had one of my fav. females turn into a male
. My SO breeds guppies and this happens to him a lot, he always has to go out and buy new females because he ends up with a bunch of males and only a few females.