Dono - Perch fry will range in size from 5.5-6.5mm long. The larger, stronger fry will be the main ones to be eating your newly hatched brine shrimp. I suspect many of the smaller fry will perish which is a good thing because you could have a hard time hatching enough brine shrimp eggs to feed thousands of fry. Brine shrimp probably grow fairly fast so the optimum size for fry to eat are the newest hatched shrimp eggs. Perch fry that are eating should also grow fast and could gain 1mm(1/32") per day.
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