1-3 teaspoons uniodized salt per gallon of water will work. Continue with the salt for atleast 1 week after symptoms are gone. Rock salt or aquarium salt. Some people say normal table salt is fine, but some say not. I wouldn't use it. Make sure it's fully dissolved before putting it in the aquarium.
This doesn't actually make "brakish," as brakish water has more than just salt in it.