For the last 6 mos with my school tanks, I use plenty of beach sand after washing a great deal, and lots of play sand too. The playsand was very "sandy" in the water for a day. I try to watch the reports about red tide and pollutants but so far I have had no problem. My tanks are very clean and things are growing well.
However...if I did go to the wrong spot or there was an outbreak of something...that could wreak havoc!!! so be prepared!