Water heated up inside the tank or under pressure (your water heater)
may cause gas emboli in fishes' gills with the symptoms you describe.
This is a bigger problem in the winter when the cold tap water has
more dissolved gas and with larger water changes. Water should be
heated outside of the tank and at atmospheric pressure with some
agitation to help de-gas the water. I have a powerhead pump the water
several feet above the container with a heater and fall back into it.
After several hours I add the replacement water to the tank over the
course of several more hours.

If there were a significant number of bubbles forming, particularly on
your heater or anything else glass, then you may have experienced this
problem.