Turtle substrates remain predominantly below the surface of the water, ruling out many types of ordinary substrate, especially wood-based ones (it would be no good to have your substrate floating around on the surface!).
However, this also rules out many different kinds of sand and silt based substrates too, which can effect things like the pH balance in the water if the wrong type is chosen, and can retain turtle waste more easily rather than it being cleaned by your turtle filter. For this reason, reptile experts have created specific turtle substrates like these!
Turtle substrates are generally larger pebbles, becoming home to plenty of beneficial bacteria which act as a biological filter for your turtle terrarium, helping to clean the water further. They won't effect things like pH in the water either, allowing you more control over your turtle's water quality.
We also sell handy-helpers like Turtle Draining Mesh, separating your substrate from the top layer, making cleaning easier.