I’m sorry if it seems I’m being difficult, but I am only concerned about your turtle's health. Every spring, I hear horror stories from people who tried to hibernate captive turtles, only to find them dead or severely weakened in the spring. In order to survive a winter sleep in captivity, a turtle must be in peak condition and all conditions must be perfect. I'll give you more in a moment.
I understand that your turtle is used to a colder environment. However, when he was wild, he was able to bask in bright sunlight. That was how he got UVB light. Sunlight provides plenty of it. UVB rays cannot penetrate window glass, so if your turtle has been living indoors in a tank for several months, he probably already has the early stages of MBD. That doesn't mean he's sick right now, but he will be on his way to it. He won’t be in any condition to survive a winter of sleep. Basking in sunlight also warms up a wild turtle enough to keep it active and eating until it's cold enough to hibernate.
The temperature in your house is not going to be cold enough to bring your turtle into a full hibernation, but it will be chilly enough to cause him to slow down and stop eating. Remaining in that condition will make him ill, and may lead to death from dehydration, fungal infections, or respiratory infections. That's why you need to do one of two things. One is to provide the optimal indoor conditions described above so the turtle can warm up, skip the winter sleep, and be active and healthy through the winter. That is what is generally recommended for turtles housed indoors.
The other is to provide the proper conditions for hibernation. In the wild, a turtle will bury itself in mud below the frost level, either at the bottom of a pond or lake, or along the banks. The temperature there will remain cold enough to enable the turtle to hibernate until spring, but not so cold that it will freeze to death. If you want to provide an outdoor pond for your turtle, it will need to be at least 3 feet deep in colder areas because if it freezes to the bottom, the turtle is likely to die. Even in such a pond, a turtle can suffocate. You can also provide hibernation conditions in a large metal stock tank, available at farm stores. To do that, you’ll need to provide a heater to keep the water from freezing solid, and aeration to keep it oxygen-rich. The heater will keep the water just above freezing, so it's cold enough, but not frozen. Proper procedures have to be followed for this to have any chance of success. A veterinary exam and check for parasites is the first step. Then, the turtle has to go without food for approximately two weeks. If a turtle goes into hibernation with food in its stomach, it will almost certainly die from a bacterial infection that will begin in the undigested, rotting food in the stomach. The food can also ferment, producing fatal gases in the turtle’s body. Prior to fasting the turtle, you must be sure that it has adequate fat stored from a summer of good eating. Otherwise, it can starve during its sleep. These are all the reasons I didn’t give you very much information on letting the turtle hibernate. If you still want to do that, you’ll have to make sure you do everything just right. Hibernating aquatic turtles is considered something that should be done only by advanced turtle-keepers, and even they often meet with failure. Here is a quote from the Happy Turtle site (a reputable source of information):
“Most temperate aquatic turtles also hibernate in the wild and this can likewise be achieved in captivity; however, it is a relatively advanced procedure requiring a good deal of specialist knowledge and experience on the part of the keeper. There is little or no room for error. Our advice in most cases is to overwinter these animals. In the wild, such turtles usually hibernate in the mud on the bottom of rivers or ponds, but even where the animals are kept in a pond in captivity, it is highly unlikely to be able to provide conditions ideal for such a hibernation; anoxia, or lack of oxygen, is only one of several possible problems which can arise. For safety's sake, unless you are an experienced keeper who is absolutely sure of what you are doing, we would caution against attempts at hibernating any aquatic turtle.”
Almost all turtle experts feel the same way. The way a turtle lives in the wild, and the way it it is kept in captivity are so different, that it is often impossible to provide them with the conditions needed for a successful winter sleep. In the wild, the turtle’s instincts allow it to choose perfect conditions. Those who are successful at hibernating a captive turtle, almost always do it in an outdoor pond. I hope that you’ll read the information on these sites:
In summary, if you want to hibernate your turtle, you’ll need to provide either a pond of sufficient depth or a tank with heater and aeration. The turtle should be examined by a reptile vet prior to hibernation to make sure he’s healthy enough and has enough stored fat to survive a winter sleep in captivity. Then he should not have anything to eat for two weeks prior to hibernation. This all has to be timed so that the proper temperatures outdoors in your climate will be reached to send the turtle into hibernation at the end of the fast.
If you don’t feel that you can provide the proper conditions for either hibernation or to keep the turtle active through the winter, a third option would be to release him near where you found him before the weather gets too cold. The turtle can then take care of his own hibernation needs. You could then perhaps find another turtle to keep through the warmer months next summer. Again, if you have more questions, just let me know. I’m happy to provide all the information I can. I hope that whatever decision you make will work out well for you and your turtle.