One of the first things you learn in this hobby is to research, research and research. I decided to mix my own salt and found several articles on synthetic salt mixes. This is just one of many I found interesting.
The article was written by Marlin Atkinson and Craig Bingman. ©Copyright, Aquarium Frontiers and Fancy. Publications Inc. All rights reserved.
Quote of the first paragraph:
"Many marine aquarists use synthetic seawater mixtures in their aquaria. The relative merits of the various mixes are often debated by aquarists and expounded in advertisements, yet there have been few published analyses of these products. Although every naturally occurring element can be found in natural seawater, private aquarists typically have access to test methods for determining only inorganic nutrients and a few major and minor ions. This paper presents a detailed and comprehensive analysis of eight commercial formulations of synthetic seawater available in North America. A refereed version is published in the Journal of Aquariculture and Aquatic Science 8(2):39-43."
Here is the complete article:
I hope you find this information useful.