An aquatic turtle's floor is a different question
Substrate advice for tortoises and snakes assumes a land animal standing on bedding; a painted turtle swims, dives, and forages along the bottom of deep water, and its floor has to be judged by what happens when the turtle investigates it with its mouth — which it will, daily. That single fact produces the safest answer in the hobby: bare bottom. A glass floor cannot be swallowed, cannot trap pockets of rotting food, and lets the filter current carry waste to the intake instead of into a gravel bed. Most experienced painted turtle keepers run exactly that, and the turtle loses nothing it needs.
Why gravel is the classic mistake
Standard aquarium gravel sits precisely in the swallowable size range for a painted turtle, and turtles pick at it while hunting for food scraps. A gut blocked by stones is a veterinary emergency, and gravel has a second, slower fault: the spaces between the stones collect shredded food and faeces where water flow cannot reach, quietly loading the tank with ammonia between cleanings. Anyone who has siphoned a gravel turtle tank knows the colour of what comes out of an apparently tidy floor. If the bag came free with the tank, that is still not a reason to use it under a turtle.
If you want a floor: big, smooth, or fine and clean
Two coverings pass the safety test. The first is smooth river rock — individual stones each clearly too large for the turtle to get its jaws around, laid a single layer deep so waste cannot hide beneath stacked piles; as the turtle grows, re-check the smallest stones against its mouth. The second, used carefully, is a thin layer of fine sand: grains too small to obstruct are generally passed if nibbled, and sand shows waste on its surface where it can be hovered up with a siphon. Sand demands discipline, though — stir and surface-clean it at water changes, and remove it at the first sign of blackened, sulphur-smelling patches underneath.
The basking end and the land question
Whatever covers the swimming floor, the basking platform should be dry, grippy, and easy to scrub — a fixed rock, a cork float, or a commercial dock — because painted turtles haul out daily and a slimy platform is where shell problems start. Painted turtles kept indoors do not need a land or digging area the way a semi-terrestrial species does; their substrate question begins and ends with the water. Outdoor pond turtles are the exception, where a soft mud-and-sand bottom is natural and safe precisely because its grains are fine and its volume is enormous.
Water care is the real weekly job
A bare or rock floor trades bedding changes for a steady rhythm of partial water changes, filter rinses, and platform scrubs, and a rhythm is only as good as the person remembering it. KinsLe's Care Teams let a household assign those rounds to named helpers, each marked complete with a timestamp, so the turtle's water stays clear because the rota says so, not because somebody happened to notice it clouding.