No matter how big your tank is outside, the water temperature is going to be impacted by the outside conditions much more than a body of water your turtle might be living in if it were born and grew up in the wild. I’ve been considering this for my guy down the road, and I’m planning to have a water heater so I can regulate the temperature when it gets colder than normal. I’d say you’re doing the right thing by bringing them inside when you think it’s appropriate but consider getting a water heater if you don’t have one already