What are Microworms?
Microworms (Panagrellus redivivus) are nematodes that are commonly used as first food for a large variety of fry. They mainly feed on yeast and can be easily cultured on oat or flour type substrates that have been saturated with yeast.
Microworms are commonly used due to its high palatability and small size. In addition to that, the live wriggling motions stimulate the fry's natural feeding behaviours. For these reasons, there tends to be a decrease in mortality rate due to fry not feeding in the first few days after hatching.
As compared to baby brine shrimp, another common first fry food, microworms are much smaller and can be consumed by smaller fry species which baby brine shrimp may be too large for. In addition to that, microworms survive much better than brine shrimp which perish relatively fast unless the right conditions are kept. In saying this, I believe a combination of the two foods is favourable for optimal nutrition for the fry. Once fry are large enough for brine shrimp, I tend to find that brine shrimp fills them up better as well.
A good article on microworms and their nutritional values can be found here: