They can survive on crushed flakes, but that is the least effective way of keeping them alive and growing.
I'm kind of old school, but anyone with breeding experience can tell you there is a universe of difference between flake and live food. First, I feed all my fish live food or real veggies. I feed flake food as well, but everyone gets live food daily. If you take a group of guppy fry and seperate them feeding one group flake only and the other live baby brine shrimp or micro worms you will see a drastic difference in two weeks or less. The fish eating live food are growing much faster, sexing out quicker, have much better color and within a month you would not recognize them as from the same group of fry as the flake food bunch. Why do I have 4"-5" swordtails? I feed live food and give them the room they need. In 8-10 weeks my sword fry are the size of those little swords you see at, "Pet Whatever." My female Endlers are 3-4 (x) the size of the female guppys in the LFS, even though Endlers are smaller fish. Besides the obvious advatnages for the fry, the adults benefit greatly as well. Many of my livebearers don't bother their own fry that much as they are too busy chasing down shrimp. My larger females throw huge broods of fry. Yes, a large female swordtail can throw 70 (+) fry. The fish demonstrate much better health and are robust in their activities. If you never tried it you should. You can get micro worms or brine shrimp eggs and produce as much live food as you with cheaply. I f you want to do an easy experiment this summer set out two containers of water. In a week or so you will have mosquito lava that can be netted out and fed straight to your fish. You will notice the difference very quickly especially if you have a group of fry 2-3 weeks old eating just flake food. Once your done, you'll be hooked.
