I read an interesting article in the AFI April 2012....about fry nutrition research-not specific to Bettas-but interesting none the less...anyway.....what the research stated was that some species of fry need live foods not just to stimulate the hunting/feeding instinct- but the enzymes and bacterial gut fauna live foods provide that they need to help them process food later in life to aid in food digestion as well as absorb nutrients and when fry food is void of this the fish can slowly wither and die even when they appear to be eating a good diet.
It goes into the importance of calcium to help build strong fins, bones, scales-they get some of the calcium from foods but most is extracted from the water through specialized pores on their body-stating the importance of water changes to renew calcium and other mineral found in the source water.
The key to knowing you are feeding enough is by the tummy...it should be bulging after feeding-if not-you are either not feeding enough or feeding the wrong food....