silica based playsand is fine. The worse thing about it is it is not the most conducive for sand dwelling things like worms and what have you, since the sand crystals themselves are jagged and not round.
I have read that but I really haven't ever seen any negative affects on them. There are many ways to do it. Aragonite, play sand, bare bottom, etc. They all have their pluses and minuses. There is more than one way to skin a cat.