I'm by no means an expert on this but I believe the min. water depth is 2 times the carapace length of your turtle. But the more water the merrier, provided that they can get to some shallow areas and a land area. Right now I have about 7 inches of water in there and my turts are 1.5" long. The main reason for so much water is that I have an Aquaclear, which is a waterfall type filter and I want to minimize the vertical drop of the water. Plus the more water there is the longer it stays clean. I provide them a large basking area (5"x8") with a ramp, and both a smaller submerged platform and a natural rock that's 1" below the water surface. Hope this helps!