Fancy goldfish gasping for air?

I have 5 fancy goldfish in a 75 gal. The water perimeters are 0 nitrites, 0 nitrates 7.0 ph. The amonia level is a little high at 0.5 so I put in some amonia neutralizr. I've only had them for 2 weeks. I found out my filter wasn't working properly and they got a case of ich. I got a newfilter the othe day. I gave them super ich cure - 2 doses and it appears the ich is gone. Today, I found one of them dead so I did a 30% water change and put the carbon back in my filter. Currently, they act like they are gasping for air, hanging at the top, and not eating. Any ideas? Thanks.