Rosie Reds are not recommended. They are high in a chemical called thiaminase which, if consumed by garter snakes in large quantities, can lead to Vitamin B deficiency. It is recommended that you avoid any feeder fish that contains thiaminase.

Thiaminase can be killed by cooking the fish, if it is fresh, but any fish rich in thiaminase that is frozen will actually be even more problematic.

Silversides and guppies are relatively safe. Frozen/thawed fish fillets from a reputable supplier are also an option. I feed Little Dude tilapia cut into strips (though Scarlett won't touch them).

Another option is Nightcrawlers. Garter snakes love them, and they are relatively easy to obtain. Your local bait shop likely carries them, as well as any large retail department store. Nightcrawlers are highly recommended due to certain biotics in their digestive tract that are very beneficial to garter snakes. If your snake is small, you can always cut a nightcrawler up into small bite-sized pieces, and if served immediately, the pieces will still be wiggling, attracting you snakes attention easily.