Gravel is not good for turtles , since they eat just about anything including substrate ! It's hard for a turtle to digest gravel and could get stuck "on its way out" :-)
Sand is much easier to clean as the poop just sits on the surface as mentioned above . I mixed some black sand with pool filter sand to make a "salt & pepper" looking substrate , it's not too dark but dark enough not to notice the poop. I also have some convicts in with the turtle so I don't ever see poop as it just dissolves in the water due to so much movement. I would recommend some lucky bamboo plants as they are great for eating all the nasties in a turtle tank .
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