I just got some goldfish flakes from walmart! Mine are on its 3 days free swimming as well. I did feed some brine shrimp.
I also bought some shrimp pellets which I make into dust by crunching them and putting them in the fry tank. I have about if 300 to feed lol I took out 50 and got bitten like crazy doing so into a fry tank. The rest are with parents.
I've had good success with pellets and flakes smashed up into small pieces. The sponge filter will pick it up and then the fry will eat it off that