I had a situation with some cories that is just a little similar. I got a 29 gallon tank and I made 3 albino corys the brave founders of the tank, and about 2 weeks later I added a bronze cory. For the first 3 weeks, they were playing around and going crazy 24/7, then they started to get lazy.
They would just lay at the bottom, eat sometimes, and as you said just come up for a gulp of air and go back to laying on the bottom of the tank. I did water changes but I just assume it was the cycling my tank was going through. The two smallest albinos didn't make it, but they went out fighting and were probably as above described for a month before they lost the fight.
The good news; my largest albino and the bronze cory that were added survived and recovered from whatever they all had, and although they don't play as much as they once did, they are always up and active, looking for fun. I'm gonna have to get them some more brothers (or sisters) as a reward for being the brave founding father of my tank =)
Anyway, that is my story and experience with corydoras. My albino has faded skin color as well almost as you describe, but as I said he is now fine, so don't put him away just yet. At least I wouldnt.