Substrate is optional here — and optional is safest
A red-eared slider's substrate sits underwater, which changes the whole calculation. Nothing about this aquatic turtle's life requires a covered tank floor: it swims, it basks on a dock above the waterline, and it forages by sight and smell through open water. A bare glass bottom gives you a tank that is easier to keep genuinely clean — slider waste is heavy, and on bare glass every bit of it is visible, settles where the filter or a siphon can reach it, and never rots unseen inside a gravel bed. Filtration works better, water stays clearer, and there is nothing on the floor small enough to fit in the turtle's mouth. For a species famous for eating its own decor, that last point is the one that matters most, and it is why bare bottom is the safest answer rather than a compromise.
The gravel problem, stated plainly
Aquarium gravel is the substrate that sends sliders to the vet. Red-eared sliders investigate the world mouth-first: they pick up stones, mouth them, crush soft ones and swallow small ones outright, and a gut full of gravel can obstruct, which is a genuine emergency. The warning signs — straining without producing droppings, a turtle that stops eating, hind legs dragging, unusual lethargy — mean a vet promptly, not a wait-and-see. This is not a rare keeper mistake; it is one of the most predictable outcomes of putting pea-sized stones in front of an animal that treats everything as potential food. Juveniles are the most at risk simply because more of the gravel on sale is mouth-sized to them, but a determined adult can swallow surprisingly large pieces too.
If you landscape: rocks bigger than the head
Keepers who want a natural look can have it safely with smooth river rocks, provided every single stone is clearly larger than the slider's head — if it cannot fit in the mouth, it cannot be swallowed. Round, smooth stones are better than angular ones for a turtle that will push them around and climb over them, and a sparse layer cleans far more easily than a deep bed, since waste settles between stones where a siphon can reach. Sand is the debated middle ground: fine play sand can pass through a slider in small amounts and some experienced keepers use it, but it clouds the water, works its way into filter impellers, and makes waste harder to spot, so it is a poor trade for most home tanks. Whichever way you go, the rule that never changes is size: nothing mouth-sized, ever.
The dry end deserves a thought too
The other surface in a slider's life is the basking dock, and it should be a solid, textured platform the turtle can haul out onto fully and dry its shell — never a pile of loose material. A dry, warm basking area is how sliders keep their shells healthy, and a dock that stays waterlogged or is built from gravel in a tray defeats its purpose. Keep the dock's surface grippy enough for claws, give it a gentle ramp from the water, and check that a growing turtle still fits on it with room to spare, because an undersized dock quietly becomes no dock at all. Write the setup down — bare floor, rock size if you use them, dock arrangement — in your slider's KinsLe profile, and Family Sharing means a family member cleaning the tank in your absence follows the same no-gravel rule you set.