If there are no visible signs of illness / disease / parasites, and you have good levels and aeration, I'd have to believe it's a contaminant that made it into the tank.
If they appear in that much distress, I'd really step up the water changes, and possibly do back to back 50% pwc's tonight. If there's something bad in the water, we've got to get it out. If you have carbon, it wouldn't be a bad idea to throw that in as well.