A word of warning about fish containing thiaminase.....
Thiaminase is a cumulative toxin, it builds up in the snakes' body. Giving a B1 supplement will not counteract the effect of thiaminase. Thiaminase destroys B1 (thiamin), making it impossible for the snake to synthesize the calcium in it's diet.
The only truly safe method is to not use fish that contains thiaminase. At all.