I have mine around 4" deep but the bottom is covered with large rocks for them to sit on so its more like 2" of actual water. Some like deeper water and will swim to the bottom but mine don't. It can't really be too deep but they will only use some of it.
Mirage came out of brumation on April 26. He was doing great. On May 2 he started acting funny. We just redid his tank, and he keeps going into one of his hides. He just lays there. He shows no intrest in food. HELP!