Use a submersible aquarium heater to maintain the water temperature
Most of the more popular aquatic turtles like Painted Turtles, Red-Eared Sliders, and Musk Turtles do well with a water temperature of about 78 to 82 degrees Fahrenheit (26 to 28 degrees Celsius) when they are hatchlings.
One more thing: Don't make your turtle's water temperature too warm. For most adult aquatic turtle species, 78 F. (25.5 C.) is the highest you want unless you're treating them for an illness and a veterinarian has told you to raise the temperature. Otherwise, too-warm water can encourage bacteria growth in the tank. It also discourages turtles from basking sometimes.
Cooling WaterWater that is too warm will cause excessive shedding and will increase the potential of bacterial illnesses. This is something to consider if you live in a warm climate and are unable to control your ambient temperature. Aquarium chillers are available, but most are quite expensive for keepers. The most practical solution may be to keep the aquarium in an air-conditioned room but out of direct flow with the cool air. Other options may be a canister filter encased in a Styrofoam cooler or ice encased in plastic bags or bottles although these ideas may only be effective for small tanks.
How to Create the Ideal Aquatic & Semi-Aquatic Turtle Habitats
Heating an Indoor Aquatic Turtle Habitat - My Turtle Cam
Aquatic turtles need the water they live in to be of the proper temperature in order for them to live. If the water is too cold, they will get sick. But if the water is too warm, they may not come out to bask enough, which can also make them sick. Your turtle needs the water temperature to be right in order to survive and be healthy.Most of the more popular aquatic turtles like Painted Turtles, Red-Eared Sliders, and Musk Turtles do well with a water temperature of about 78 to 82 degrees Fahrenheit (26 to 28 degrees Celsius) when they are hatchlings. Once their carapace (upper shell) is about 1.5 inches (3.8 centimeters) in length, the temperature can be gradually reduced, a degree or so every few weeks, to about 74 to 76 degrees Fahrenheit (23 to 24 degrees Celsius). Caring for your turtle is relatively easy once the basics are established. Dispel the thoughts of getting a turtle and putting it in a bowl of water with a rock. Those little "Turtle Ponds" you can still find in pet stores, are completely inadequate and have even been dubbed by experienced keepers as "Death Bowls". Turtles require more than a small, simple container to ensure proper health and longevity. Heating, lighting, temperature control, filtration - all play an important role in the health and happiness of your turtle. Even a large aquarium, filled to the top with water is not meeting their needs, as they need an area to get out of the water and get completely dry.Most pet supply stores sell a wide variety of heaters that fit under a tank, or which are shaped liked rocks and go inside the tank. I don't recommend any of them. They're designed for terrestrial reptiles, not aquatic turtles. In my experience, the under-tank heaters are expensive and useless for regulating water temperatures, and the rock heaters usually can't be used underwater.