I've had river rocks (the kind you get in the crafts section at Walmart) in my tank as a substrate for several months now. Since I've finally upgraded to a 10 gal, I want to go ahead and get 3 panda corys. I'm a little concerned though since I know their "barbs" are sensitive and people typically recommend sand for them. I really don't want to change to sand if I can avoid it. The rocks have been tumbled so they have a smooth surface. Will they be okay?