Just do water changes for now.
If you want to add some salt you can but the fish should get better with clean water. If you do want to buy some salt, get rock salt (aka aquarium salt, swimming pool salt). It's sodium chloride without anything else added to it.
Avoid table salt because it usually has iodine and anti-caking agents in it and these can harm the fish.
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The dose rate for salt is 1-2 heaped tablespoons of rock salt per 20 litres of water. Keep the salt in the tank for 2-4 weeks then stop using it.
When you do water changes while using salt, you need to add salt to the new water before adding it to the tank so the salinity (salt level) in the tank remains stable.
After you have used salt, do a 10% water change each day for a week using dechlorinated water that contains no salt. Then do a 20% water change each day for a week using dechlorinated water that contains no salt. After that you can do bigger water changes.
The small daily water changes dilute the salt out of the tank slowly so there is less stress on the fish.
Having said this, do water changes first and save salt for treating sick fish. Don't add salt unless the fish are sick.