With Onn (getting a bit long in the tooth at 9 months in the tank), I have recently tried skinned, deboned pieces of salmon. Not a great choice sustainably but my local Publix will cut me a 1" by 6" strip at the seafood counter (laughing at the purchase) and it seems to be successfully replacing the tougher shrimp that he has started rejecting. Be very careful to remove any bones as there have been reports of octopuses found with fish bones impaled in the brain and some of the bones I have found would seem up to the task.