im sorry, but i think you have this backwards... they get far more nutrition feeding on mice then on fish... they mostly feed on fish because they cant constrict and kill mice... fish should be a treat while mice should be the main diet... no question about it... there is also no need for vitamin suppliments if feeding a rodent diet.... but with fish it is necessary as in the wild they would have a varied diet like frogs salamanders etc... just fish would lead to malnutrition