Marine or aquarium salt?

It’s very important to understand the distinction between a salt mix for marine tanks (which is what we want to use in a brackish set-up) and aquarium tonic salt. Aquarium tonic salt is simply an aquarium-suitable form of sodium chloride (it won’t have any additives that could make it potentially toxic) that is occasionally used to help treat certain conditions such as fungal outbreaks, or to help with osmoregulation in fish recovering from ulcers or other chronic stress issues. Meanwhile, marine salt is a mixture of all of the elements of the sea, including the likes of magnesium, potassium, calcium and even rarer elements like gold and arsenic. Many of these elements are essential to the physiological functions of fish that have evolved to live in seawater, and tonic salt will not provide them.