Just an FYI...
The white caribbean sand I found at a local Walmart was called Gulfcoast. However, it's not aragonite sand like Southdown, it's regular silica sand. If in doubt, you can take a small container with vinegar and put a sample of the sand into the vinegar. If it fizzes, it is likely the aragonite type and is an excellent choice. If it doesn't, it's probably silica based and can still be used, but would be a second choice.