I think you have gotten great advice so far.. and have done everything right.
Sometimes new fish have health problems prior to you getting them, and it's hard to diagnose a new fish's illness since there was no symptoms leading up to it that you would of been able to see.
Your advice has been spot on, what I would of recommended. Thanks for bringing this thread to my attention, I will keep an eye on it.
When he goes up for air, does it seem as if he is having trouble swimming? Like struggling hard, possibly tipping ever so slightly? The head down position I don't like, is that how he is constantly, or just for the picture?