Hello and welcome. Thank you for requesting me. If your area has a lot of sunshine during the summer months, your cooter may be just fine outside. That's because when he basks in the sun, it is warmer than the ambient air temp would lead us to believe. Your idea of using an outdoor basking light is fine. If the enclosure is spacious enough, your turtle
can move into or out of the heat as he feels like it.
As long as he is active and eats well, he is warm enough. If he slows down or loses his appetite, it might be time for the outdoor basking light. Monitoring his behavior and appetite are the best ways to know if he is warm enough.
There are a couple of other problems you may encounter with outdoor housing. One is that if your tank is actually in the ground (like a pond) your turtle is likely to simply leave. They go off seeking mates the first chance they get. A small fence a few feet out from the pond will be needed. Many people make a rock or concrete block wall just tall enough to keep in a turtle. Predators often injure or kill turtles that live outdoors. Raccoons are the most common offenders, and they will chew off feet, and destroy shells. Dogs will grab a basking turtle and carry it off, thinking it's a toy. This site I has some tips to minimize these problems.
If you have more questions, just let me know by clicking on REPLY. I wish you success.
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