Well...if they are getting crowded, you'll have to take at least a few out. Those may not survive in the main tank...but you don't want all your fish dying. Usually you take them out when they can't fit in the biggest fish's mouth. Sometimes a little after to prevent bullying.
Is there any extra tank you could use? Or maybe divide off a small section of one of your larger tanks for them?
Using a divider. Usually you can find ones to fit any tank at your fish store. But if it's only a 15, you may have a little trouble doing that....what other fish do you have in your tank? And how old are the fry?
Hmmm...how big are they? The only fish I'd really worry about munching on now would be the Betta. Usually once the fry hit a few weeks, they can outswim the parents.