Is goldfish a common noun

Yes, the Word 'goldfish' is a common noun; a Word for any goldfish of any kind, anywhere.

A proper noun is the name of a specific person, place, thing, or a title; for example:

• Goldfish Street, Corpus Christi, TX or Goldfish Way, San Diego, CA
• Pepperidge Farm Goldfish Crackers (a registered trademark)
• "Memoirs of a Goldfish" by Devin Scillian and Tim Bowers
• "Goldfish have no Hiding Place" by James Hadley Chase