I was wanting to know the best diet for a garter snake.

I would like to know if a only insect and fish diet would be a suitable diet?

I somewhat don't feel comfortable feeding it pinky mice. I can't even finish off a rat that managed it's way into a rat trap. Even the deseased wild rats are cute to me. x_x

If that is the only healthy diet. I wont be getting a garter. No point in getting one and feeding it a awfull diet. =D