Kurt said it well.
1 & 2 you've got right.
3, Calcium carbonate will help buffer your water, but you don't NEED to use calcium carbonate based sand to have a successful tank. IMHO
, it definitley helps.
4, You can use an all silica based sand if you wanted, but a CaCO3 based sand would be better, IMO
. The thing about silica leaching into the water and creating diatom problems...I don't buy it. Glass is mostly silica and it hasn't been contributed to causing diatom blooms. Again, CaCO3 helps to buffer, and can go a long way in keeping pH up if other conventions fail (such as tap water, salt mix, or whatever).
I'm 99% sure Kolorscape sand is perfect for use, even though it probably is a 50/50 calcium carbonate/silica mix. Someone correct me if I'm wrong, but I do believe that all playsand must be sterilized before it can be marketed as such. This should mean that the sand is safe for use. If you do a google search for kolorscape play sand, you'll come up with a bunch of threads on reef forums and people using it in their tanks. Only thing I've found so far that is detrimental about it, is that it can be sharp. But unless you are going to be stocking your tank with garden eels or sand sifting gobies, I wouldn't worry about it (and I probably wouldn't even worry about it even if you were).
Just my opinions, of course.
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