Tank Girl is right on track. a good rule of thumb is: the fry are ready to be introduced in the main tank when the smallest fry will not fit in the largest fish's mouth. That will keep them from being eaten. I would definately have plenty of plants or other hiding places in the tank to protect the fry while they are still small and feed them crushed flake food. The fry know better than we do when it is safe to venture freely in the tank.