After getting over our shock, I took some water samples and we whipped through a quick cleaning and water and filter change. Forensic ichthyology revealed a giant spike in lethal nitrates, so I ran down to the fish store and bought some chemicals to neutralize them. Hate going for the quick fix, but we're leaving again tomorrow morning (for the road trip to Kansas to visit kadyg's family that we've been planning for months). The emergency work seems to have stabilized the remaining fish, but the loaches are still swimming oddly and I really don't know what's going to happen after we leave again. (UPDATE: Nitrates still ~15 ppm, will do 50% water change and one more round of chemicals before we go ...)
To compound the issue, most of the ammonia (that bacteria turn into nitrates; and subsequently into nitrites, which the plants filter out of the water) comes from either fish waste or uneaten food. The culprit was probably a combination of heat plus the zucchini I left for our pleco to munch on in our absence: looks like the vegetable disintegrated much faster than expected, and rotted into waste material that hit the tank all at once. I added some beneficial bacteria, which will only have a delayed effect, but I guess we'll just have to put the fish on a starvation diet until either I or a roommate can monitor the tank more closely. That'll be at least another three and a half days.
I'm not sure how many of them will last the week. What a time for this to hit.