It should be noted that rosy reds, AS WELL AS goldfish are fine for garters; As long as they are part of a varied diet.

And ALL cyprinids, which includes all minnows and most other common baitfish, contain thiaminase.

If you want to be really anal about keeping thiaminase out of your snakes, tilapia, salmon, and trout are supposedly thiaminase free; but then again, seeing as these are available primarily in filet form, your garter misses out on the nutrients it would be receiving from fish guts, eyes, stomach content, etc.