If he was anything like my tom clown he'll survive in the same water that beta fish survive in. For reals though, I had one survive when he was floating upside down gasping for air (MY mistake) and when all the other fish croaked, he make a complete comeback. YMMV, but that's one hardy fish.
Just keep a close eye on him, if it get's worse, get him into a QT/hospital tank.
Good luck!