I've had mine since they were TINY, and they paired up over 6-9 months. They always stayed together from day one, but it took some time before I saw the "mating dance" so to speak. The male was doing his shakes constantly for weeks and then one day, the female decided to start hanging out with him at night when the lights went off. Now they mate constantly. Once they're old enough, paired up, and comfortable with their surroundings, it should be fine. Mine tend to spawn more often when rotifiers are added to the tank for some reason, maybe they think that's the best time since there's food for the babies, or something.