If I understand correctly, thiaminase is a chemical that destroys thiamine, a necessary nutrient. It is in fact very highly present in goldfish, and with any diet that is not already rich in thiamine, it can be extremely unhealthy - even deadly - to consume.

Thiaminase - Wikipedia, the free encyclopedia

I'm not a snake expert by any stretch of the imagination (in fact, I only came here to learn about Thamnophis because I had a bunch of them in my yard), but it sounds to me like this pet store "expert" really isn't much of one.