On a goldfish only diet im not supprised you've head stories of snakes dying
, goldfish are a no no realy, as its been said before they are not nutritionally sound enough. (and I think they contaion thiaminase, I may be wrong there)
Good conditioned mice however generally have everything a snake needs and need only be fed once a week,of course you can supplement the diet with other things.
I see no reason why a snake should suddely die from eating mice.