If you want to save the fry you have to watch closely, look for the mother 'standing' on her mouth, possibly twitching like she's having contractions, then very gently move the mother from one tank to the other (with matched water conditions) and put her in a breeder trap (your LFS will show you) so that the babes will drop through. After she's done, you move her back to the other tank...
You're actually SUPPOSED to move her a week before she gives birth...but that takes practice on the timing, and I don't even know what that is. I'm assuming that because I found my fry 3 weeks ago tomorrow, I'm moving her today. Still working on the DIY breeder trap...