I wouldn't just trust that the water is fine because you did a water change, have canisters and the other fish are ok.
Some fish will perish from poor water quality and/or low oxygen levels before others. Some fish just handle it better than others. And a 50% water change doesn't fix things if you have high levels of pollutants, you're only bringing those down by 50% then.
So until you test your water parameters to completely eliminate the chance of that being the issue, no one can really guess at other possibilities. You also need to test pH.
It may just be an issue with that particular fish, or some kind of disease, but nothing you said completely rules out water or oxygen issues, which are the major causes of the symptoms you described.