Not all salt mixes are created equally. Each manufacturer uses different concentrations of certain elements in their formulas. In general, most salt makers will alter their formulas so that the resulting saltwater mimics natural seawater. However, certain parameters are intentionally elevated to provide better conditions for your aquarium livestock. These include calcium, magnesium, alkalinity, and several others. Unfortunately, undesirable substance might also be elevated, such as nitrate, phosphate, and silica. On top of that, aquarists have to deal with low concentrations of beneficial elements, which is obviously unintended, as well as the occasional bad batch. With all of the salt brand out there, it can be difficult to find the best one. My only suggestion is to poll other aquarium keepers and experiment on your own. Find the brand or blend that works best for your situation.