Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Sand can be from any of several sources.
If the parent material was limestone, calcite, dolomite or related materials, the sand will add minerals to the water, and raise the pH.
Many sands in the building trades are silica sand. This is safe in the aquarium, but does tend to encourage diatoms for a while. Comes in many grades. Usually is off white, somewhere from cream to tan. Can be fairly bright white, too.
Pool filter sand and many others has been graded so all the particles are the same size. The bag will be labeled. These are the best. With all the particles the same size water can flow between them, bringning an exchange of O2, CO2 and ferts through the substrate.
Play sand is usually not graded, and can have a lot of finer particles. This can pack down when used as a substrate, and contribute to anaerobic pockets.
If you buy play sand, and start washing out the fines you may wash away as much as half of your purchase.