Keep her water shallow. 2-3 inches. Don't worry about
the water heater. You really don't need it if the room is
kept around 70 degrees. With shallow water, your
basking light will warm it during the day and allow
a cool down naturally at night much like being in the wild.
I have never used heaters on my turtles, even hatchlings.

Baby turtles can drown if not given easy access to
basking areas when in deep water. The baby should almost be
able to touch the bottom and have his head above water.

Hope this helps. Here is one of my turtles raised from a
hatchling. She is 9 years old now.

Linda