Not 100% true, although studies aim to prove both sides of the issue (that a goldfish diet kills garters, and that it doesn't).
product."

If you read this carefully, you'll see that the problem was feeding frozen fish, in which the thiaminase concentrates (so I'm told). Heating is supposed to destroy it like this guy says.

There are other garter breeders who feed primarily minnows and goldfish and have no problems through Nth-generations of offspring. My garter subsisted on "primarily minnows and goldfish" for almost 8 years with none of the symptoms mentioned. Many others can corroborate.