Maggie's Mom wrote:
I have a solitary goldfish, possibly a comet. I had him for about 6
years now, and he is about 7 inches long, plus the tail. He lives in
a 30 gal. tank, with all the amenities a goldfish can possibly need -
and he appears sad.

My question is: would he enjoy a company of another goldfish?

sure, they love company.