In my experience the parents start getting pretty beaten up at three - four weeks, but since you've only got 20 fry they may not damage the parents so fast. I'd leave them for a bit more, maybe 4-5 weeks, keep feeding bbs, and if the parents start looking like they've been in a fight, move them out. By the way, I've found that freeze dried black worms is a great food for young fry, I use a coffee grinder and grind them up fine, them churn them in a small bowl of aquarium water with a small wire whip. Then as they age move them on to different food. Good luck.