Jkessler623;3973389; said:wats wrong with gravel?
If its small enough gravel, the turtle could end up eating/swallowing/choking on it trying to get food off of the bottom. If the stones are all too large to be eaten, that is fine, but again, those will collect detritus a lot faster and be more difficult to clean.
With sand, the detritus collects on the surface, where a good current can help get it suspended in the water column, to be removed by the filter. Also, sand is less dangerous to the turtles if ingested. And they can dig in it.