The sand in the link you posted is not "Southdown" and is likely silica-based sand. Sand sold under the name "Southdown" hasn't been sold publicly for years. CaribSea and other companies that sell sand for aquariums, basically offer the Southdown (or technically the parent company of Southdown) more money then they sold it to Home Depot for. Home Depot in turn just buys silica-sand from other companies, as to them, sand is sand.
I have not seen calcium-based sand sold as "sand" at any of the major home improvement chains in years. What some of them do have, is Lime Fertilizer, which is calcium-sand. Though they are usually extremely dirty and need thorough cleaning before use, lots more then even silica-sand.
Which I will hit on, as it's been mentioned already. The only real issue when using silica-sand in your saltwater aquarium is aesthetics. Silica sand will not release silicates into your water. Silicates are extremely resistant to being removed from the sand, and the chances of silicates from the sand going into the water, is just as likely from the very glass (silica-glass) that the aquarium is made out of. You have two options with silica-sand, that yellow "general" use sand, which sort of gives a weird freshwater look to your tank. The other is that white play-sand, however once in the tank with saltwater lights on, it really looks like concrete. Not very attractive IMO.
Using silica-sand does have a few issues, mainly how dirty it is from the store. The play-sand is a bit cleaner, but it still needs to be rinsed repeatedly before putting into your aquarium. The yellow stuff should probably be screened to get tree branches and rocks out, then heavily rinsed. As silica-sand is void of calcium, it'd also not able to buffer your water the way calcium-sand can. However there's a side debate to whether sand buffers the water in the first place.
