Its mainly because it offers no buffering capability as aragonite (calcium based ) sand does. The beaches on the gulf side of Florida and such are basically all silica (quartz) based sands......I have used it for years in some setups and it does work ok, if you have a lot of live rock ., or just a FO tank.
Pure silica, apart from the buffering issues, is not a problem, IMO. But silicious sand from the beach tends to have additional mineral impurities that may not be desirable, IMO.
I've heard aragonite sand really doesn't provide really any buffering anyways. In order to actually buffer, your pH would need to be way outta the ideal range.
Any truth to this?
...I picked aragonite just cause it was at the pet store and looked nice. Use quickcrete in my sump and looks good as well; just to light and thin in my opinion.
Calcium carbonate based sand definitely helps buffer your water; it dissolves in our systems at such a slow rate we never notice it.
Your pH would have to be really low for you to notice your sandbed disappearing and if it was that low there would not be anything living in your tank.
The other thing I've read is that silica based sand has very sharp edges wheras aragonite is very smooth. If you have any fish that sift the sand, it will cut up their gills. It's also not good on snails or other critters that would move along it.