Comet-weird behavior

Hi, I am new to this group. It is cool to know that there is a group
out there like this. Anyway, I have a question about my comet
goldfish. He seems to be having issues getting to the bottom of the
tank, like he has to work extra hard to get down there. As a result he
spends most of his time at the surface of the water. I have seen him
turn to his side a couple times, kind of like he is bobbing back and
forth. I am not sure what to do. I am really worried about him! Any
suggestions as to what I can do will be appreciated. Thanks!