I'm not sure you get what thermoragulation means...

Reptiles are cold blooded. They move back and forth from warmer areas to cold areas to control their body temp. In a small cage, such as a 5 gallon, there is no cooler place to escape to. The turtle only has access to one temp and can't cool off. You need to provide a thermal gradient.

Do you have a basking spot?

I think you should move your turtle into a ten gallon and provide a thermal gradient and try feeding other prey items such as live tubifex or bloodworms. The little guys, in my experience, have more of an appetite for these tiny live worms than anything else.