Not all aquarium salt mixes are created equal. Each brand sports their own formulas that deliver different concentrations of all the marine aquarium essentials, including things like calcium, magnesium, and trace elements, among several others. Generally, most salt mixes promise similar things, but not all of them perfectly deliver on those numbers. As a result, there is a great deal of variability in the quality of the salt mixes and what they actually bring to the table. This problem is exacerbated by the fact that there can even be some variability within the same product, as different batches come out slightly different than their predecessors. For aquarium keepers searching for stability in water parameters, these issues present significant problems. Fortunately, with diligent water testing, regular water changes, and a strict dosing regimen, the quality of the salt should have a marginal impact. Just be sure to perform water tests on newly made batches of saltwater, prior to your water change.