Whats wrong with my turtle!?

I have an alligator snapper, he lives in a 30 gal tank with two red eared sliders and 4 large goldfish. Today I found him, and his body is swollen, there seems to be a lump in his throat, and he is laying with his neck stretched out not really moving but sometimes appearing to gasp for air or swallow something. I don't know what to think except maybe he swallowed something he shouldn't have? I don't know what it would be in the environment he lives in. Any ideas will be appreciated thank you.