A water change is definitely necessary considering the amount of food you've been feeding them. Fry is more sensitive to water quality than adult fish in most cases. With fry.. most of what you feed them will be uneaten... we always have a tendency to overfeed fry, which is why it's good to start with live foods.. so they stay alive long enough to be eaten. Betta fry have a crucial period of needing to be fed after 48 hours, if they're not fed after this time when their yolk sacs are depleted, they tend to waste away even if they feed at a later stage. Why this occurs is still unexplained. So, water change(with conditioned and aged water) and continue feeding microworms and if you can, provide some freshly hatched brine shrimp.