Thanks for the advice everybody! I will definitely be switching back to aspen for the majority of my snakes.

It's such a pity though, the peat-moss is so attractive and my hognose slides into it as if it were water. It's so cool to watch. However, the health of the snake is priority.

I am concerned about using aspen with my garters though. I once dislodged an inch-long piece of aspen from a vent and it just didn't seem like a good idea after that.

Would unfertilized potting soil be okay? My snakes were wild-caught as adults, would bacteria still be a problem? How problematic is bacteria in substrates? I mean, it's everywhere and they're bound to be exposed to it. Are there specific types of bacteria found in soil that are more dangerous?