If they eat fish food, the size doesn't matter. I still feed mine 2mm pellets and they gobble them up despite being too small for some of them. The sooner you get them on fish food the better. As a note, mine never liked prawns much at all, neither are they crazy for mussels. They go crazy for bloodworms. And any fish would go for live food such as black worms, bloodworms, etc... in case they refuse fish food and dead food. However, ideally they need to start feeding on high quality pellets to get all the nutrition they need.