Don't worry! A few minnows will not kill your snake instantly. If you can find guppies, they are great for your snake to eat. Also, you can use cut up pieces of salmon or trout as long as you sprinkle on some calcium powder. You could also mix in nightcrawlers for variety. It's not necessary to feed your garter snake rodents, as they are fish, frog and worm specialists in the wild anyways. Some larger garters have been shown to take rodents in the wild but rodents are not a garter staple in the wild and they don't need to be in captivity if you don't want them to be. They can be, but it's definitely not a requirement. Nature knows best and you can't better it!

If you can find silversides, I highly recommend using those. They are a long skinny silvery fish often sold at pet stores frozen, as food for fish. Snakes love them and they are safe for snakes to eat just as guppies, salmon, and trout are. My entire collection of garter snakes gets silversides as the main food, with mice and nightcrawlers and frog legs mixed in occasionally.