Tokis-Phoenix
^_^
Neither, to be honest. You should use an effective med rather than salt to treat ills.
When it comes to setting up a brackish tank, marine is far superior to tonic.
Salt can be effectively used to treat gill flukes, although i agree using a med is the best option, certain fish (like mormyrids) can not always be treated with certain medications (mostly those containing stuff like copper) for certain ills, so salt has to be used instead.