So it appears that fish aren't the only dead things in our backyard.
Last night we were getting ready for dinner, and Paul looked out the kitchen window to find a dead pigeon in our waterfall's pool.
Paul lifted it out with a shovel, I dug a hole under one of our stepping stones (heavy thing on the top in case some opportunistic neighborhood predator chanced along) and we dropped in mister deadbird. Then we both washed our hands like surgeons. Paul said that when he moved it, it was all stiff and dessicated, meaning that it'd probably been dead for a while and something'd dragged it into the backyard. We see the neighbor's half-feral cat back there a lot, so this is not an implausible theory.
Two days ago I went back to the petshop and had our water tested (I made a post for it but must have hit "Preview" instead of "Post", because it apparrently never went live, so here you go...) and everything checked out as normal: chlorine, pH, ammonia, nitrates, nitrites, and a couple other things. Everything. The petshop girl confessed that she had no idea why the fish were dying. She sold me another three orange comets at ten cents apiece. If these three die, we're getting mosquito dunks and being done with it.
Last night, I saw two fish, this morning I saw one. The countdown to chemicals begins.