You should track down aSnakeLovinBabe.
She has so many garters! Here's a little article she posted on another forum a while back:
Feeder goldfish, and also their counterpart, rosy reds (aka tuffies, flathead minnows, and remember these come in both pink and brown colors) and catfish contain an enzyme known as thiaminase. Feeding your snakes these fish will result in horrible things down the road. it can even take years, or it can take just a few months, but thiaminase kills either way. This enzyme prevents the absorbtion of Vitamin B1, also known as Thiamine. Over time, not being able to absorb this vitamin due to ingesting fish containing thiaminase, they will start having seizures, crawling funny... and this eventually results in death. Sadly, I expierienced this myself, as when I was younger and had no idea, I too fed my garters goldfish. None of my first garters are alive today because of that, and they should be!!
Many ask if it's acceptable to supplement these fish with a vitamin powder loaded with Vitamin B1. The answer is no, because the enzyme literally prevents the snake from absorbing any at all!
There are other options out there if you snake is finicky and wants fish, and beleive it or not they are at you local supermarket! Trout, tilapia, and salmon are all commonly sold fish, you can buy a pound or two and cut it into minnow-sized strips, and it usually comes out to about 2 cents a peice where as most people pay 10 cents or more for one dinky feeder fish. If you fish... save some bluegills and cut them up as well. You can then portion all the pieces out into meals and store them in your freezer, and thaw out a new portion whenever you need to feed!! Freeze all of it for at elast a week before use to kill any parasites that may be lurking! Not only is it cheaper in the long run, but it is saving your garter's life. Note: don't buy catfish, they also contain thiaminase! You can use feeder guppies if you are having trouble getting your snake to eat dead pieces of fish. Put the fish strips in with the guppies in the water bowl and as the garter is fishing, he will inevitably grab a strip. But please... make sure they are GUPPIES... some pet stores will give you rosies when you ask for guppies! See the pictures at the end of the article.
I must also stress though, that if you have a garter snake, a varied diet is a must!!! Most of us expierienced garter keepers wean our garters as babies onto pinky parts, easily done by chopping up fish and worms and pinkies and mixing it all together in a little dish. Over time you increase how much pinky is in the dish and they pick it up very quicky! I find that many adult garters will take a pinky for the first time out of pure curiosity. If not, usually all you have to do is rub the pinky on some fish or a worm! The best possible diet for your garters is a mixed diet consisting of primarily small rodents (pinky mice and fuzzies if theyre big enough to eat them), Safe fish (salmon, tilapia, trout), and eathworms or nightcrawlers. (don't use red wrigglers, those little trout bait worms that have a reddish tint and rings and secrete nasty orange slime, they are toxic). Remember to dust a meal every so often with some vitamins!
I hope someone out there finds this information useful... because if you are feeding your snake dangerous thiaminase containing fish, you will stop if you truly care about your snake! As pet owners it is our responsibility to see that they get the very best care and a proper and safe diet, you wouldn't slowly poison your dog, would you??? Wild garters don't eat goldfish. The responsible thing to do is to drop the feeder fish idea and save your garter's life while you still can.
NOW here are some pictures to help you identify what the heck I am talking about.
These are feeder guppies. they are SAFE for your snake.
Male:
Female:
These are Rosy Reds, a.k.a "tuffies", fathead minnows... etc... they are NOT SAFE for your snake.
Gold variety
Brown
These are Goldfish.. the most commonly sold feeder fish. RUN FROM THEM. They are NOT SAFE.