I'm well aware of those poor hatchlings in NYC's Chinatown and environs. A year ago (about) I was walking along Canal St. and it was freezing. They had those poor things swimming in little bowls of water out in the wind.
The sad thing is that you were kind to try to save two of those unfortunates, but there were two more that took their place. Buying them only perpetuates the business.
Because they're kept in such awful conditions, many are sick when bought and don't make it (like your other turtle?)...
If he was swimming normally until just a few days ago, I'd keep an eye on him for a few days and see if he doesn't go back to a more normal position.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-