I've just had something similar happen, and it's been a frustrating thing to figure out. I think I've resolved it so, If I share my experience, maybe some parts of my story will help. Also bear in mind that I've just recently returned to the hobby after 20 years.
I was running a Fluval U2 in an 18.5 gallon tank. I had 8 bronze corydoras and they suddenly started flashing after I introduced (gradually) 10 pygmy corydoras and a hillstream loach. So that probably validates your decision to quarantine. I started losing fish; first the pygmies (which clearly arrived sick) and then my bronzes.
I treated for ich and later dosed with a praziquantel based medication. The latter initially caused more flashing (I'm assuming the dying flukes irritated the cories). Things slowly got better, but 4 of my surviving cories had lost a lot of weight and most of their barbels. I kept losing them slowly. Some of the cories put on weight but remained lethargic, clearly looking unhappy.
I was about to give up, but then I watched and read a lot about aquarium flow. The time was coming up to redose the prazi, and in the meantime, I removed a rock and a 12 inch plant that was right in front of my filter outlet and I got all obstructions out of the aquarium corners where debris was gathering in dead spots. 1 or 2 days later, my cories were active again.
I then added 2 airstones and, about half a day later, the bronze cories were darting about at high speed again and looking better than ever. All flashing had stopped. I didn't remedicate at that point and there has been no flashing since. They also hardly ever gulp at the surface interestingly. It's like looking at an entirely different tank!
I don't know the science behind this, but I would bet that my poor water quality in the dead zones and inconsistent oxygen distribution was stressing the fish, thereby making them more susceptible to parasites and the illness that had hitchhiked on my pygmies.
So you're very probably onto something by considering your water quality.
