Basically you dose enough salt to get the water to about 1 tablespoon/20L after than you just leave it be... anytime you change your water, If you want to retain the same level of salt, you will have to add salt to the fresh water so that the concentration is the same.
One thing to ask though, its not clear whether you are trying salt as a prevention or as a cure for something? Maybe you could elaborate why you want to use salt? and also what kind of tank you have.
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