Many people are able to get their betta fish to spawn despite not doing things ‘by the book’. These spawns are most often quite small because the fish aren’t properly conditioned. I have heard from a bunch of people who email asking about what to feed their betta fry because they set up a spawn without having any baby fish foods ready. My first answer is usually to try microworms because they are nutritious and the baby Siamese fighting fish are attracted by the constant movement of the microworms.
Microworms are a type of nematode (tiny worms). They are cultured on some sort of starchy substrate (it doesn’t really matter, but I prefer oatmeal; others like bread). Microworms have the essential minerals and are a good source of fatty acids, proteins, and sugars for growing fish!
If you are interested in trying microworms, it is recommended that you get them a few weeks before the spawn is planned so that your starter culture can be ready with enough worms to allow you to harvest from it on a daily basis. A good site is http://www.buymicroworms.com