keep the water temp in the 80s-90s...unless your soaking under the sun. Then, you can go with cooler water temps...cuz the sun with heat it up! Also, just enough water to cover 1/4 the height of the carapace. With my hatchlings...literally 1/4" to 1/2" of water! As yearlings, the ITs have about 3/4" - 1" of water when they soak!
Yep. Around 90 is good for mine. 85-95 is what I shoot for, taking into consideration if I think the water will get cooler (indoors in winter) or warmer (outdoors in summer).
I use an aquarium heater in my Box Turtles soaking pool. It keeps the water temperature within 1 degree of my setting. I made the pool out of concrete and sealed it with marine epoxy. It is 10 x 10 with a sloping bottom to 4 inches. Max is only 4 months old, so for now the heater sets under a false bottom with the water level 3/4 inch in the false bottom. I also have part of the pool covered so he can hide under it. He loves it.