This salt question gets funny.. Who makes what salt and for whom is a big part of the answer. There's a only a handful of companies that actually make the salt, with the major branders sending them their trace element blend to mix in.
Up until a few years ago, Coralife made Kent's salt. Kent's is now made by Instant Ocean with a few ingredients thrown in, which I believe are Coral Vite and Zoecon(maaaaaaaybe)
Seachem wanted Coralife to make their salt, but Coralife never had the capacity. Since the Kent switch, Coralife now provides the base for Seachem's salt. Seachem adds trace elements and buffer. Infact, Seachem as far as I know is the only company that actually adds buffer. Salt mixing naturally forms sodium bicarbonate(NaHCO3 pH 7.8~), so most manufacturers rely on that as the buffer. This is one of the reasons it needs to be mixed thoroughly before use, as NaHCO3 changes to Sodium Carbonate (Na2CO3 pH 8.8~) once in the water long enough. Eventually it will balance out something close to natural levels. Seachem adds Borate (BO33- ph 9.2) for extra stablity within the carbonate blend, which should settle around 8.4.
Red Sea's claim is that their salt is made from evaporated sea salt, making their calcium more bioavalible as it's not the pharmaceutical grade everyone else throws in their mix.
Oceanic has high calcium, but that's offset by high aluminum and low carbonates, making it bad with buffering.
This study is a little old, but it still gives a good foundation of knowledge, and will help keep this post shorter... and my advice, go with what works. If you're not getting the results you want, start trying new things slowly.
http://www.advancedaquarist.com/2005/11/aafeature1">http://www.advancedaquarist.com/2005/11/aafeature1</a>
[IMG]http://www.advancedaquarist.com/2005/12/aafeature1/view">http://www.advancedaquarist.com/2005/12/aafeature1/view</a>