water+sand+bacteria=live sand. Your fish guy is an idiot. Having said that. Sand+LR
+bristle woms= less detritus. Less detritus=better water. So again, your fish guy is an idiot.
Sorry, I've had a long day. Anyway, if you have live rock you are going to have any number of hitchikers. Bristle worms, spaghetti worms, pods
, etc. Bristle worms are good, fire worms are bad. Fire worms however are fairly rare, so generally not a concern. Your hitchhikers will migrate down to your sand over time and make the sandbed healthier and more efficient at what it does. You do not need to buy "live" sand to do this, it will just happen. "LIve" sand that comes prepacked is just sand and water. A very expensive way to fill your tank.