All good advise, above there, but I'd like to emphasize a few points. First, male garters often (not always) go off food during the mating season. Don't know when, or if, you brought it out of brumation, but this may be a factor. Second, I'd get the fish out of the enclosure. Constant presence makes their water a breeding ground for bacteria and fungus. Hopefully, you've been cleaning the water dish regularly before replacing the fish. Third, you might want to double-check on the temperature in the enclosre. If it's too high, the snake will be distressed, and if it's too low, it won't digest properly and will refuse to eat. Keep us posted on its progress, so we can tweak the recommendations to suit your unique situation. Best,

Rick