Zebra danios need a lot of room to breed. They are egg scatterers. When they are breeding, the males and females swim rapidly the length of a good sized tank with the females dropping eggs along the way and the males following and fertilizing them. You will seldom see any fry unless you are breeding intentionally because when they are done breeding, they don't hesitate to come back and eat any eggs that are accessible. Many breeders use large gravel or even kids glass marbles as a layer on the tank bottom so the adults can't reach the fertile eggs. They also lower the water level so that the eggs don't need to fall as far to reach the safety of the marble bed.