Here is a part of an article I have been working on about changing substrates, you will find a list of what silica sand can include. If you find a bag that lists ***silicates*** then you do not want it. Silicates lead to diatom blooms
So what sand do I use to build my sand bed?
The answer here is aragonite sand. Many hobbyists have found that Southdown, Yard right or its apparent new name of castle sand ( or something like that) works very well for reef aquariums. This is due to it being calcium carbonate based, and with an excellent grain sizes makes for a good functioning sand bed. 1/8mm is very fine> 1/256mm to 1/16mm is considered silt, and less that> 1/256mm is considered clay.
Southdown seems to mostly contains very fine to silt size particles, and maybe a few larger than 1/8mm.
Other aragonite sands are available through many LFS. If it is aragonite it is good.
The trick here is to make sure that it is aragonite and not calcite sand as many different minerals have identical chemical formulas but wildly different properties. Aragonite is orthorhombic crystal and calcite is rhobehedral, scalenohedral or prismatic crystals
This means they have a different arrangement of atoms giving them different properties of density, solubility, hardness etc. They also tend to incorporate different trace elements, based on what best fits into their different crystal structures. Aragonite will substitute larger atoms such as strontium. Calcite grabs magnesium and iron. Aragonite’s properties are much more beneficial in a closed system.
It also seems that many have used Quick Crete play sand with success though it is not calcium carbonate based, but silica based instead. No this does not mean that you will have silicates in your tank.
Very Generally speaking Silica sand, lets say industrial style is a high purity quartz (SiO2) sand. Silica is a stable compound and will not leach silicates into your tank for it has no "sillicates in it. Depending on its chemical and physical characteristics, silica sand is used as glass sand, foundry sand, abrasives ..etc. Silica sand can contain tiny amounts of impurities, such as iron, manganese, chromium, calcium, or aluminum, and give the sand its color depending on how much of these impurities are contained within. So it depends on geographically where your silica sand comes from as to how much impurities it contains. If you are going to use silica sand look for white silica sand as it is much more aesthetically pleasing
A warning about silica crystals:
Silicosis is a disabling, nonreversible and sometimes fatal lung disease caused by overexposure to respirable crystalline silica. More than one million U.S. workers are exposed to crystalline silica, and each year more than 250 die from silicosis. There is no cure for the disease, but it is 100 percent preventable if employers, workers, and health professionals work together to reduce exposures.
So in other words do not breath this dust. FYI there is also a warning label required by the state of California that is required on silica play sand.
Ways to test sand to see if you might be able to use it in your aquarium:
Take a sample of your sand and test some of the sand with vinegar. If it bubbles/dissolves, it is calcium carbonate sand and should be safe to use.
Take some sand and pass a magnet over it or in it, it might come out with little metal shavings from processing, if it does I would not use it. Though I have heard of some who have with little poor effects. Iron can actually be a fertilizer for macro algae.
Hope that helps some
Thomas
