The Gold Fish is a hard-to-find, rare fish which is strictly for selling purposes. When you get one, you can get a whopping 717 gold from merchants. It will go to your fish inventory page as any other fish, which is very convenient as it won't take room in your item inventory. You can also stack up to 5 of them.
A convenient glitch with the Gold Fish allows for the easy creation of infinite gold. If your Hero has a Barter level of 10, you can sell the Gold Fish to any Merchant for 1,433 gold, and buy it back for 1,147 gold, resulting in a profit of 288 gold every time it's repurchased. Take caution to not sell it to a Merchant with a full 50/50 inventory, or it won't appear in their stock to buy back.