what type of sand to use?

Sand type is not so important as making sure the sand is cryo'd first

What are the risks to the planet if you mix Alien Sand with Machina Dynamica Brilliant pebbles?

No,no,no.Do not mix them.That stuff is quacky.My Alien Sand is the real deal.Beware of false prophets.Stick with proven,real life acessories.Only 5 pounds left,act now.

Play sand mixed with small gauge lead shot should do the trick nicely. The differing density of the two materials mixed together deaden the sonic signature of the rack without emphasizing or de-emphasizing any part of the frequency band thus not allowing the stand to have a sonic signature of its own.

Quicksand.

No one has talked about fine grain sand. Just go a janitorial supply place and buy a 50 LB bag of silicone sand (used in hotel lobby ash trays). Cheap, about $10-$15 a bag. I used it with Bright Star Audio Big Rocks when I had them. Good listening, Jeff