I once got a batch of 100 fry but they don't sell microworms in
Singapore. I only had BBS so I fed them that. However only 8
survived but they were all very small even at the age of 5 months.
are a riddle that has not yet been solved. Many questions and
sometimes few answers... That is, until I come into the picture LOL.
But seriously, it is often hard to figure out betta problems. As a
matter of fact, the President of the IBC and I were powowing
yesterday and picking each other brains about some growth issues.
Why do some strain grow faster than others? Why do some grow fins on
small bodies, while other bettas get huge fast but their fins don't
catch up until much later... Why do marbles have the reputation of
being slow growers? Is it true? When it comes to solving betta
mysteries I rely heavily on my own observation and that of other seasoned
breeders I know. So here is my two cents on your question:
You can raise a spawn on BBS alone and theoretically they should get bigger faster on BBS than on microworms. However BBS is heavy to digest and excess of it causes many swimbladder problems. Swimbladder does not kill fry so this is not why you lost most of your spawn. In short, not feeding microworms and feeding only BBS could not have cause your fry to not grow nor cause them to die. Hence you need to look at other factors. I suspect either poor filtering, overcrowding, or poor water quality or not enough food or not enough water changes is what caused your fry to die. Another thing to consider: If the fry become sick (velvet, fungus or bacterial outbreak), many will die. The remaining ones will be weakened by the sickness and usually that will stunt their growth as well as affect their finnage.
So if I were you I would go back to the drawing board, read this site section on breeding thoroughly, and try to determine why diseases wiped out my spawn. Then try again! (Never give up! Never surrender!!)