Well , my advice is if this is a newly purchased goldfish, then it's most likely not anything you did wrong. I would just take it back to the store where you bought it and ask for an exchange with another goldfish. This probably the cheapest way, since it will probably cost more time and money to try and treat it.

Also one thing you should be aware of is that barbs and betas are tropical fish species , while goldfish are cold water species. They prefer different water temperature ranges in order to stay healthy so it's usually not a good idea to mix goldfish with other fish species.

Gluck

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