When did you do the water change? Is his tank filtered? How big is his tank? Have you tested his water for ammonia, nitrites & nitrates? That should be the 1st step. If you don't have a test kit I would do a water change & see if it makes any difference in his behavior. Mine sometimes gulp air like that but they don't hang at the top much. When you changed out the gravel & did the 100% water changes its possible you lost a lot of BB so now you're having a mini cycle given your tank was already fully cycled. I may be completely off base because I don't your tank set up.