How can you tell if your minnow is male or female?

fish from pet stores are invariably underfed and somewhat skinny, which makes them harder to sex. Also it seems as though a diet rich in live food (brine shrimp for example) is really necessary to get them into prime breeding condition. After a few weeks on shrimp the belly on the females will swell up and the males will look better (more golden, esp. the ventral fins) and you will easily be able to tell them apart. The male and female zebrafish can be distinguished by two cues, color and body shape. The males are slender and the females are more rounded. The males have a pinkish-yellowish cast and the females a bluish-whitish cast. I have seen pet store zebras that look more like pencils than fish. In that case it might take some fattening up before you can tell.