If you can help it, also know where your "safe" fish comes from. Just about any fish you can buy at the store, has varying amounts of PCB's and/or mercury and therefore isn't safe to use as a staple, or at all, regardless of being thiaminase free. Mercury poisoning is a real concern not just for the snakes, but for us too. I honestly wonder how many cases of "thiamine deficiency" were actually cases of poisoning by heavy metals.

List of Seafood Health Alerts - Seafood Selector - Environmental Defense Fund