should i use any kind of substrate since he likes to dig?
No, it's not necessary. Actually having substrate in a turtle tank is a pain in the butt to clean. Just go with a bare bottom or if you really want something use ceramic tiles or slate.
should i use any kind of substrate since he likes to dig?
I would argue that you still need to take care while cycling, whether its a turtle or a fish.
You'll want to keep the water shallow, about 2-3 inches deep while it's this small.
should i use any kind of substrate since he likes to dig?
I've read that it needs to be at least as deep as the turtle is wide so that it can right itself if it gets on its back---turtles can drown in water that is too shallow to flip back over. Is that correct?
The Austin's care sheet says "They are excellent swimmers and their captive habitat should have a water depth as deep as possible." Does that not apply to hatchlings? (Sorry---painted hatchlings like deep water and that's the only kind of turtle I have, so I don't know about other species.)