I don't know if your filter running with the kink in it could have any kind of effect on the oxygen in the water. Hopefully someone else will take a look at this too. I don't see any of the usual suspect issues that might result in fish breathing heavy. The water change should help with that at least in the short term. I'm wondering if a cut back in filter flow can reduce oxygen in the tank water? If it were me, at this point I'd try to increase the tank aeration. You could use an airstone if you have one, you can lower the water level a couple of inches to create more surface agitation, you could even run an empty HOB on the back just for the extra flow and water movement.
Just one more idea, think back to the last time you used your hands in the tank water or during food handling....could you have had something on your hands that wasn't rinsed off good enough or....as I did accidentally one night....use a knife to slice a veggie when the knife had some medicine residue on it? Go through the routine you did. Any chance anyone else in the house could have put their dirty hands in the water?