I've never used this product, but purely in terms of what I know about chemistry:
Silicon dioxide is quartz. It is completely insoluble, and not the same as the silicate jsoong mentioned. ALuminum, Magnesium and Titanium oxides are also insoluble.
Calcium oxide will dissolve somewhat. Calcium makes the water hard, which may or may not be good depending who lives in the tank. This will also tend to raise pH without adding any KH
, so I would be careful of it for that reason. However, carbonate in the water would remain available as a buffer against acid formation, which is the normal concern in an aquarium.
Potassium oxide will dissolve for all practical purposes completely. It is a very strong base and will have a large and temporary effect on pH and is therefore unacceptable in a tank. Thorough rinsing should remove it.
That leaves the iron problem. Jsoong is correct about staining your tank orange, as well as about the algae and sharpness of the particles.
I wouldn't use it, as I don't think there is really a ready solution for the iron, even if you decided to go with all guppies or something that wouldn't care about edges. A pool/spa store like Algenco mentioned would be another good source of black sand, or a gardening store or nursery.