Which Play sand is unusable and why

[ocellaris_keeper]

I want to use a lot of substrate in my new tank (4-6 inches) and want to use the play sand as the base.
Which ones can I NOT use and why?
What makes the sand unusable?
Can I use pool filter sand and why not?
Why can you use Southdown Play sand and not another brand?

 

[glowplug]

why use playsand? There so much other stuff

 

[seannmelly]

for saltwater, you need to use sand that is calcium based to buffer your ph. most pool filtering sand and play sand is silicate based which will not buffer your ph and lead to problems. also, i have heard of users that have had horrible algae problems using silicate based sand.

 

[ocellaris_keeper]

so the silicate based sand causes unstable conditions.
Why?