Some basic requirements that are not above standard practices can prevent skin blisters, including keeping a warm basking spot and clean water. The best preventative is keeping low levels of salt. Use aquarium salt for 2-4ppt. Don't know specific gravity. If they already have them, keep low levels of salt and dip them in strong concentrations, such as 20ppt for ~20min.
If they persist, a culture will need to be taken at a vet.
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"I know the human being and fish can coexist peacefully."
Governor George W. Bush, Jr.
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Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)