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They'll eat flakes. I feed my crays a 50:50 mix of sinking commercial catfish pellets and rabbit food. They're omnivores that do well with a diet of both animal and plant proteins. Also provide a calcium supplement by way of cuttlebone in the filter or crushed oyster shell (bird grit) in the gravel. And, provide some iodine in their tank to aid in their molting. Regular drug store iodine (couple of drops per 10 gallons) or invert. iodine will work.
Never introduce copper to the cray tank (decor or medication) since copper is fatally toxic to most inverts.