Both are correct. They both mean 'occasionally' or 'now and then'. 'Every once in a while' carries a suggestion that the activity described may happen slightly more often than 'once in a while', but it is a minor distinction.
Fish should be fed twice daily in the morning and evening. Give them as much as they can eat in 15 minutes. Any left over food should be removed immediately to avoid it going rotten and poisoning the...