It is possible that you pH shocked the tetra. But the black phantoms, like many tetras as a rule, are comfortable on the acid side, at least down to 6, though maybe not all at once.

By the way, I have a hunch that Melafix, from a tea leaf plant, also acidifies the water. That may be one of its uses, at a pH of 5 bacteria cease to reproduce. They may be inhibited above that.

Increasing the oxygenation is good so long as the fish aren't given more current than they can deal with. If everything is sheltering in a still place, behind a large rock, the current is too great. ;)

I have shocked really touchy fish with a large water change, but as a rule water changes should improve their condition. I'm sure you didn't temperature shock them. There are other ways to throw them into a chemical shock but unless a pathologist were to do a post mortum on your fish within minutes of death, many times you and all of us really may not know the cause of death. And even a post mortum may not always provide answers. :)