You need to feed them more variety. Baby fish need lots of different food to grow well.
eg: When the fry were about 3-4 weeks old, I use to feed them on crumbled up dry food, then marine mix that was cut up really finely, then bloodworms that was cut up, then microworms, then newly hatched brineshrimp.
That was one feeding, which included all those foods. The flake was used to give the greedy fish something to eat. The marine mix was the main part of the diet that filled up everyone. Any fish that was sill hungry got bloodworms, which they all managed to eat. Then microworms, which are small and easy to swallow. Then newly hatched brineshrimp that would swim around the tank for the next couple of hours and the fry could eat them when they wanted to.
The fish were fed until they were full, and then the newly hatched brineshrimp were added. All the fry had fat little stomachs and looked like pregnant guppies. The idea of filling them up is the food takes longer to digest when they have a full stomach. This means they get nutrients over a longer period of time.
I fed them that about 5 times each day until they were mature. At which time they were either sold or fed less often.
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Don't mix different aged fry because the big ones eat the smaller ones.