I've always read that anything that your turtle can put in his or her mouth shouldn't be used in the tank as decoration or substrate. I used larger fish gravel when they were really tiny and now I just use medium to large river rocks.
@1981camaroz28- That's interesting to know about the aquarium sand, I've always been curious about using it. How do you clean the sand? or do you just toss it?
I read on a site a while back that sand was bad to use, but it was a site dedicated only to hatchlings. I don't know, baby humans always poop sand after a day at the beach and it doesn't seem to bother them, haha.
Oh PS, it's REALLY easy to use just river rocks, it looks nice if you get enough to cover the bottom. I use a cyphon 1-2 times a week to suck debri from between and under the rocks. The ph levels are fine, and I only do a complete tank cleaning once a month.
You could try it, but I think if you're turtles are healthy and happy with sand, just keep it.