Frozen mice are NOT of lesser quality. They are not less nutritious either.

As for the fish, IT DEPENDS ON THE SNAKE! Garter snakes eat fish, ball pythons eat gerbils (In the wild), Eastern Hognoses eat frogs, Mud snakes eat sirens.

So, if all you care about is "natural", I suggest you start feeding live gerbils.

So, depending on the snake feeding fish can be very healthy. Feeding a garter snake mice would probably result in death by fatty liver disease.

I disagree with feeding live because it is less humane for both animals involved. I have fed live in the past, but I ALWAYS stunned the prey. It's just common sense.