Garter snakes have a very high metabolic rate thus burn foodstuffs rather quickly. A adult should be fed a appropriate sized meal every 3 to 4 days. They really don't digest fur very well so pink mice or pink rats is a safer choice. Sally's frozen San Francisco Bay brand silversides are a safe fish against thiaminase and endoparasite loads. If your snake is wild caught be advised its already living with a high parasitic load and stress from recent captivity.