My gold fish has brown underneath its belly :(\
Our New Goldfish Has Some Brown Spots On Its Body And Fins, What Could It Be?
It could be a fungus. I would talk to a pet shop that deals with fish on a regular basis, they would know, without you having to take the fish to the vet. It could also be a disease of some sort, not that that would be too far from a fungus. Hope this helps, good luck.