These are BBS that you have hatched yourself from eggs? They are only nutritionally viable for about 24 hours as is in the hatchery. After that you'll still have some that are alive, but nutritionally speaking they're pretty useless, and for baby fish you want proper nutrition. If you stick a batch in the fridge, they should be okay for an additional 24 hours or so, but you may have issues with die off. When I was breeding bettas, I had two hatcheries going for BBS (the cheap DIY water bottle version). I made sure to get a new batch going every 24 hours so I constantly had a source of nutritionally viable BBS for the betta fry. If you have too many, call it a write off and just hatch much smaller batches.