How to help my goldfish to get better?

In the past, my goldfish Fabio has had a bladder disease where he floats upside down. To cure this, I soaked his food in water to make it sink so he was not swallowing air. This helped, so I kept doing this. Recently, however, he has been sitting on the bottom gasping. This has been happening for about 3 days, so I changed the water yesterday. This has not helped. I am thinking that this may be a reaction to the new food I have been feeding him for about a week, but I'm not sure. Please tell me what to do or what site to go to for help. Thanks!