I had read somewhere that you don't want to use play sand since it tends to introduce silicates and other stuff into the tanks. Any truth to that? I'm planning on flipping my 50 from crushed coral to sand in a couple months. Needless to say $6 a bag is more appealing than $25
From my reading and understanding, "Play" sand is sterilized, and by law should not have any silicates do to the fact they should not be inhaled, let alone by a child at play.
Do some more reading, and always do the "Vinegar Test" this shows the sand is calcium carbonate based. Among other things. Most all the guys here on RC who setup DSB or huge systems with sand use Southdown.
I also use Randy Holmes-Farley's 3-part Calcium / Alk / Mg supplement, that uses Epsom Salts, Calcium Chloride (Dow Flake / Ice Melt), and Baking Soda! So, all I can say is read up, learn to save your money.
In general .. there are two types of sand ... calcium carbonate and silica based sands. Both of those types of sand come in a variety of degrees of textures .. from thick (gravel/crushed coral - to super fine). In many cases the texture of the sand is more important that the chemical composition.
Calcium carbonate sands are suppose to have the additional benefit of buffering PH since these sands will dissolve when Ph drops adding alkalinity to the tank. While technically true that benefit is overrated since that process takes place so slowly that its basically meaningless in the context of protecting anything in the tank.
The issue with silica based sands is the myth/issue/question of whether they generate water soluble silicates which in turn create algae issues within the tank. That depends on the type of silica based sand your acquiring and sometimes that's an unknown. If your getting ocean sand then that sand has been exposed to water/elements for a long period of time and water soluble silicates are long gone ... same goes to a lesser extent with many land based silica sands. However some of the commercial sand sold in hardware stores isn't really natural sand but crushed/pulverized rocks which may have a greater tendency to leach silicates into the water column.
Whats available in your local is unknown to me ... your reference to metric weight would indicate your along way from anywhere I shop.
I would go with aragonite personally because the life of sand in an aquairium isn't that long. Sand will deteriorate at about 2 inches a year and for that I would rather have something that will supplement calcium into my system. i heard the SouthDown is a useable sand but that story froma friends experience turned into a silicate haven and helped build alot of nuisance algae.
Its kind or like the rocks you buy for fresh water tanks, you can get the two lb. bags for $3 each or you can go to the garden center and get a 50lb bag for $3.50 each. The only differance is you have to rince the rock from the garden center a lot better. Sand is the same way the sand at HD is just not garanted to be clean for the reef tank but if you rince it till its clean it should be fine. Only my thoughts on the subject.