Southdown (Old Castle) sand is aragonite sand. You CAN NOT buy it anymore. The playsand you can buy at Lowes or Home Depot is a silica based sand.
SILICA IN REEF AQUARIUMS by RANDY HOLMES-FARLEY, Ph.D.
"One of the issues that has been floating around the reef keeping hobby for a long time is the issue of whether “silica” sand actually releases soluble silica or not. It is remarkable that so many people have strong opinions on this issue, and yet so few people have ever bothered to do the easy experiment of measuring it. Many even fall for the trap of concluding that since their glass aquarium is not dissolving, then silica sand must not be either."
"In the first experiment I took 3 cups of sand, and suspended it in 3 gallons of freshly made Instant Ocean salt mix that initially contained less than 0.8 mM of silica (0.05 ppm
SiO2). After 48 hours of gentle stirring with a powerhead (the water was stirring, but not the sand), the silica concentration had risen to 17 mM (1.0 ppm SiO2).
I then rinsed the same sand 5 times with 1 gallon RO
water (1 minute each time), discarded the contents, and then ran the same stirring experiment with 2 new gallons of Instant Ocean salt mix. In 48 hours the silica concentration had again risen, this time to 15 mM (0.92 ppm SiO2).
Then I let it sit unstirred for another 96 hours, and the concentration had risen more, to 23 mM (1.4 ppm SiO2).
In a different experiment, I took about 45 pounds of sand, and added 2 gallons of Instant Ocean salt mix. I let this mixture sit for 7 days, with once a day mixing with my hands for about 30 seconds. At then end of this test, the concentration was 90 mM (5.4 ppm SiO2).
It has been suggested that the amount of silica coming from calcerous sand might actually be as high or higher than that from silica sand. To test this hypothesis, I repeated the small-scale experiments above on a calcium carbonate sand from Home Depot (Southdown).
In this case, there was some soluble silica released after the first 48 h, but only 1.6 mM (0.1 ppm SiO2), or about a factor of 10 lower than the silica sand. In a long-term test, the concentration had only risen to 5 mM (0.3 ppm SiO2) in 14 days with once a day stirring.
From these experiments, I conclude that:
1. The “silica” play sand that I purchased from Home Depot can substantially raise the dissolved silica concentration in seawater.
2. The dissolvable portion of the silica sand cannot be completely removed by several rinses with either fresh or salt water, although it may be decreased somewhat by that process.
3. Southdown calcium carbonate sand (likely aragonite) can release soluble silica, but about ten fold less than the “silica” sand.
Is it OK to use silica sand? Probably. Many people do so. I also believe that not all “silica “ sands will be the same for the reasons described above relating to processing of the sand and the nature of the mineral inclusions present. So the fact that many people successfully use some (or many) types of silica sand does not necessarily imply that all people can use any type of “silica” sand without a problem."