One of the most popular sicknesses among goldfish that I know of is white spot disease, commonly called ich (said like "ick" and it is an icky parasite). If you're not familiar with them, you can take pictures or the tank itself to a vet. Or just call, most likely. There's other ails goldfish can come down with but that's a common one. If I were you, if you wanted to try again, I'd do a very thorough cleaning of the tank first. Use hot water, maybe salt, sterilize it before you give it a new inhabitant.
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If I were you, if you wanted to try again, I'd do a very thorough cleaning of the tank first. Use hot water, maybe salt, sterilize it before you give it a new inhabitant.