I think this is what's going on... http://www.fishforums.net/index.php?...oms-explained/
I did vacuum the gravel this last water change, and oh boy was there a ton of decaying plant matter and poo in there. I uprooted some of my plants in the process, so I don't know if I killed them but at least I got a lot of gunk out of the gravel.
Basically it seems my husbandry was pretty bad, and it resulted in this explosive problem when I added the two snails (I'm guessing). And because I was using those useless test strips I was unaware I had a problem at all. The snails, as Cris said, were probably the tipping point.
Poor fish, I feel terrible.