Yes Stefan. Amphibians are one of the other "primary" complete nutrition sources, but in addition to fish, baby birds are occasionally taken. I think that rodents are often "chosen" over fish because they produce less watery and smelly feces, and don't have to be offered as frequently. I wouldn't want to suggest that a fish diet isn't complete or adequate, though... as long as people feed the right kind of fish (e.g. lacking thiaminase). Occasionally supplementing with vitamins/minerals is simply good advise regardless of the diet. Personally I treat worms as a treat, and a laxative in case the snakes haven't pooped lately
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Rick