The only time you should ever use aquarium salt is for treating illness.
Goldfish needing a tonic effect can be salted at 1 teaspoon per gallon for a long time.In fact some people permanently keep goldfish in this solution
Salting a tank at .2% is commonly used to treat ulcers, other flesh wounds, fin rot and as an aid to help them breathe better through a nitrite spike. This level can be tolerated for a few weeks to months.
I know someone who has their tank salted at .2% for the past 4 or 5 months.
Salting to .3% helps to kill off external parasites and the goldfish will be happy with this level for 3 weeks.
Salt can only be used though if the fish is happy and healthy to begin with. If a fish is fighting ich and is less than two inches long and looks unhappy, then salt isnt advised as it may cause kidney damage.
If you need to salt then add 1 teaspoon per gallon or 1 tablespoon per 5 gallons U.S, which is 3 for Ukers every twelve hours until the desired solution is met.
If using to treat ich then add the salt first before upping the temperature afterwards.
All salt except table salt can be used, even pickling and kosher salt.
I have no idea what salinity .1% would measure on one of those scale things for saltwater, probably not a lot