Most "play" sand is silica sand. Most people look for old castle, etc. because it is aragonite sand and will give you better buffering capabilities.
I have read that you can use silica sand as long as it is 100% silica but some silica has feldspar and other not so good ingredients. These can be harmful to sand critters due to the particles being sharp.
I am not saying you can't use it because seems like some do but most will try to find those particular brands.
I spent the extra money whan I did my and got it at the LFS
. $22 for a 30lb bag sucked but I could not find anything better. I only have a 75 gal
so it did not hurt real bad