So, an update to the update.
I don't think that it was the temperature swing, although that may not have helped.
No, what I'm 99.9% sure happened now is that in starting my new nano as a rescue tank I rushed things a bit. I knew that my 100g system was going to collapse in the living room because the (pretty decent quality, but still from the LFS) tank stand was deteriorating, with a piece of veneer'd MDF swelling that was the primary support member for the front left corner...
...ANYWAY...
What I'm 99.9% sure happened is that I saw diatoms in the tank, and I wanted to nip that right in the bud because they're ugly and I wanted my daughter to enjoy our nice pristine new tank with our beautiful fish in it, so I jumped right on that stuff with PhosGuard, Purigen, Poly-Filter, and activated carbon.
Are you guessing what happened yet? This was actually a new theory/concept to me because I've been off on my own maintaining my tank without forums and without my local club for a decade. I've been too busy (and frankly, too broke) to keep up on the state of the art.
So, two things, really:
1. I got a new really powerful LED light (SmatFarm) cheap that could grow some corals in my little nano some day, because the stock Fluval EVO X lights were ugly and dim
2. I started ultra-filtering the tank because I wanted the diatoms to go away
So what happened then?
I got dinoflagellates. I got a tank swarming all over the place with dinoflagellates. They're on my rock, they're on my glass, they're on my filter media and sand, and they were in my fish's eyes and gills. And I haven't ID'ed them yet but I'm pretty sure they're the toxic sort.
SO, I now have an empty tank. Total wipe today while I was at work. I lost EVERYBODY in a couple of hours. I even tried a 30% water change during work hours (WFH) and still lost a fish an hour until I'd lost everybody.
So, I've removed almost all of my filtration. I still have the polyfilter in because I threw out the stock mechanical filtration the tank came with. I'll probably buy a replacement. I'm going to let the tank sit fallow for 2 or 3 months and do what I can to wipe out the dinos. Right now with the filters out, I'm feeding the empty tank to try to get the non-dino flora going again.
This experience really sucked. At first I was SURE that I had been right a few days ago and that little file fish really did have flukes, but no, it turns out I rescued him from his nightmare experience in a lost FedEx box for a weekend only to suffocate him in a tank full to the brim with bad dinos.
I lost fish I've had since before my 12 year old was born.
Never would have even guessed that over-filtering would be a thing, honestly. I don't have any corals atm so wasn't concerned with keeping nutrients up.