[Thamnophis]

Before reading all the reactions I want to say the following:

Maybe they eat a lot of goldfish, but these have belly's filled with natural food (I presume). In this is Vitamin B1 present, amongst others. And a frog every now and then also helps.

If you see no snakes in the wild that seem to suffer from thiaminase means there is no problem.

Also in captivity there are not that many snakes that suffer from thiaminase. This are still exceptions.

I am convinced that thiaminase is not such a big problem in general watersnakekeeping.
But it looks like some specimen are more sensitive for it than others. And maybe some species more than other species.

I have personally never had a problem with thiaminase. And I kept Thamnophis, Nerodia, Xenochrophis, Rhabdophis and Natrix for over 35 years. And I never give additional Vitamin B1.
I only use multivitaminproducts (where some B1 is in).
I feed smelt, chickenmeat, and some fishfilet (Pangasius ypopthalmus, Clarias lazerna) and every now and the a pinky or jumper mouse.
I do not heat the fish. I thaw the frozen fish in roomtemperature. Sometimes in lukewarm water when I am in a hurry.