Did the guy who told you that keep a straight face? Silica (aka silicon dioxide) is the major component of sand... at least here (no so in Hawaii, but that's maybe where he told you that story?!)
That was a good one. Anyhow, I have play sand in my tanks, and it works great. No problems with dirt in it, since I have Malaysian Trumpet Snails (Melanoides Tuberculata) in my tank. They keep the substrate free of organic matter, and they aerate it doing so. No tank should be without them! Also, sand is the best substrate for plants.
Just one thing: Look at the sand closely. Most sand originates from old river sediments, i.e. sand pits. The sand will be round and perfect for bottom dwellers. If the sand has sharp edges, though, it comes from crushed rocks. That is not good for fish, since they can get cuts, especially catfish. In that case, just look at another home improvement store. There are so many.
Have fun, sand is a great substrate!
HarleyK