All you EVER wanted to know about Southdown

[WaterKeeper]

There are some myths about silica sand. They tend to promote the idea that it leaches silicates into the water column. That is not true and they are no more soluble, for the most part, than the glass panels in one's tank. True they do have sharper edges than aragonite sands but not enough so to be a concern. Even with true aragonite, its contribution to maintaining alkalinity and Ca levels is not all that great. I prefer aragonite but see nothing wrong with silica sands.

[clowntang]

Somewhere in this thread I mentioned about Southdown,
and I still have 6" of it in my reef, and about 3" in the sump
.... well for over 5 yrs at least.

[Travis L. Stevens]

I prefer aragonite but see nothing wrong with silica sands.

Agreed. Now, with that said, I have my doubts that the silica in the sand (SiO2) and the glass of our tanks (~70%+ SiO2) is completely inert in saltwater. But, when and if glass/quartz releases any usable silicates, it will readily be taken up by other lifeforms, most likely without the aquarist ever noticing. Now, I contribute the original blooms of any algaes (micro, macro, diatom, etc) to the purity of the sand itself. Often "play sand" (and even aragonite or crushed limestone) is pretty nasty and dirty, and the nutrients and other chemicals that are in it can quickly be taken up by lifeforms and visually seen by the aquarist as a "bloom". Once all this stuff is taken up, you'll rarely see any blooms again.

[WaterKeeper]

I've always attributed the diatom bloom in a newly setup tank to be caused by encrusting sponges dying on the LR and releasing soluble silicates into the water column. I've never attempted to quantify this however so it is speculation on my part.