The issue with water depth is if you get it too shallow where if he gets himself flipped over he can't do that turtle flip over maneuver and he could drown. This is even more likely to happen if you don't have any substrate and he's just on smooth glass. You want to have enough water that he can't get stuck on his back but not so much that he's swimming constanly to get air. Deep is OK if (like I think Steve said) there's lots of stuff to climb on right up to the surface. It ain't cool to wake up one morning to find your new hatchling dead on his back in an inch of water.