Does the quickcrete sand have silica in it?
Yes it does. While Southdown is Caribbean, Quickrete is silica. Some people think that Quickrete causes leech but I am not sure. Although if that causes leech, then your tank glass should too
Since silica is sharp, it may not be comforting home for lots of beneficial stuff. For example, I read about an experience when hobbyist noticed that only the layer that had livesand he bought fill with bacteria but nothing even going close to his quickrete layer. But I don't have personal experience. In my opinion, if you can't find Southdown (which I can't either), invest some more and get reef sand from online (look for good deal somewhere).
But as you will read, many are happy with Quickrete and will advice you to get too.