Once again, the pet store dude appears to be misinformed. Most of the people on here have a few large river rocks on the bottom of their tanks. They aren't tightly packed and the turtles do love to push them around and do it all the time. It doesn't do any harm to the tank and it keeps the turtles entertained.
On turtles' natural habitat, there's a gallery out there with lots and lots of photos of wild RES from all over and you can see that their prefered habitats are muddy rather than rocky. They bask on logs or on a muddy shore. Here's a typical photo:
Not that there aren't rocks in their habitat - because there certainly are - but they don't predominate. It's also a fact that we've seen over and over that when there's gravel in the bottom of the turtle tank, eventually the turtle winds up eating it and sometimes it causes huge problems. So why take a chance?
