I wanted to use pool sand in my new tank, but when I got to the pool supply store, they had instead riversand. Darker, nicer to my taste, I got a sample in order to test it for shell fragments. The content of them was very low, a couple of bits in a pound. Still I decided, as I read in some page, to treat it with acid (muriatic, which is chlorhydric acid solution). The worst idea. It has been four days since and I cannot get rid of a yellow tint every time I rinse the sand, over and over. I let it rest overnight and the next day, water still comes out with that hedious yellowish tint.
It seems the acid attached to the microscopic pores of the surface of sand grains. I have yet to boil it, as it usually breaks down superficial bonding. Although it is becoming lighter, I fear for its possible toxicity.
I strongly recommend not to do the acid treatment. I would rather use one part of distilled water to counteract the hardening effect of shell fragments, if it is really necessary (even worse, it wasn't for me).