Betta fry are difficult to breed. One of the hardest things is, well, what do they eat. Don’t try to use powders or flakes, it will just totally dirty the tank. The best I’ve used are microworms. These are little wavy worms that you can grow yourself and never run out of them. I bought my first microworm culture from a site called aquabid. Make sure you get this culture before your betta fry are born. Like probably a week before. When the fry are free-swimming and ready to eat, it’s time to feed them these microworms.
What I use is a mashed potato mix with a consistency similar to peanut butter. I just used potato chips and water and put them in a small clear bowl with a lid with little holes in the top. Place a tablespoon of the culture you got from aquabid in the center of the container on top of the mashed potatoes. After about a week you will see them crawling up the sides of the bowl. It sounds a bit gross, but it’s not that bad. You can then take a Q tip and run down the sides of the bowl removing the microworms. Then just pour them into the Q tip tank. You will be able to watch them move and watch the fried betta babies eat them. Do this twice a day.
It is good to have 2 to 3 microworm cultures growing at all times. All you have to do now is repeat the instructions above and you will never run out of food for your betta fry. The cultivation usually lasts about a month or so, so always make new mashed potatoes and take a spoon of the one you grew and start another.