Daily water changes will speed up growth.
Also, 40 fry is a lot. What size tank do you have them in.. A bigger tank will speed up growth once the fry get to a certain size.. If they are in an undersized tank, water change as much as possible, maybe even more than once a day.
Also, see if the fry will eat something larger than baby brine shrimp.
See if their bellies are stuffed after feeding.
Thanks so much for the feedback....you said that I can move them to a bigger tank....they're in a 10 gallon tank right now with a sponge filter and heater....so you think I can move them to a new tank with a backpack filter at this point? I also have another set of fry that are about a month old in another 10 gallon tank....can I now combine the two in a bigger tank with a better filter?
Good luck.