There are a few methods that can be used to treat lice infestations in goldfish. One option is to use a parasitic treatment, such as an ichthyoparasiticide, which is a medication specifically designed to kill parasites on fish. Another option is to use a salt bath, which can help to dehydrate and kill the lice. It is important to note that whichever method you choose, it is essential to follow the dosage and application instructions as directed by the manufacturer or a veterinarian. Additionally, it is important to maintain good water quality and to thoroughly clean the tank or pond to prevent re-infestation. It's always better to consult a veterinarian or an aquarist with experience, as the method may vary based on the severity of the infestation and the species of fish.