I agree with you Blue. By all means I'm not trying to downplay your warnings. I don't like the idea of store bought fish, and many are salted... and of course this potential toxicity.

Snipstedy, I think your critter might have a lot of problems. Gartersnakes are not freeze tolerant, so if it got down to 20F there can be a lot of damage done. Further, if you are using a 150 watt ceramic heat emitter then you have an imbalance issue in your tank. It's going to dehydrate the enclosure, likely over heat it etc. A better description on your husbandry might better help us help you.