After I noticed the odd behavior, he went largely immobile and settled down under/behind a rock above. I checked on him a couple of times in the meantime and he was still breathing, but wasn't making any improvement. I checked him again this morning and he seems paralyzed caudally behind the pectoral fins. His back was locked into a curl and he had no pitch/roll control when I prodded him into open water. I treated him to an ice bath. The other cory and neons in the tank are fine as are the 20 to 70 pond snails.
I'm thinking he may have ingested some stone fragments or something (small snail) and was impacted. I'll get the remaining cory some more schoolmates this weekend at the LFS
and see howthat goes.
Oh, his caudal region had atrophied a good bit, he looked more tadpole-like than fishlike from above over the last couple of days. Catabolism? and maybe amplified by a swollen abdomen. . .
odd. wish I had euthanized him earlier, but held out hope that something would literally pass and he'd get back to normal.