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Thank-you for requesting me. I have been a Vet tech for 30+ years with special interest in Reptiles and reptile rescue.
I think you pinned it down very well. D3 Toxicity will cause Neurological damage. It will cause the twisting and turning throwing the head back leading to lack of appetite. I never recommend D3 only plain calciu without D3 or Phosphorus. Now he needs to be Detoxed. He should have some fluids from the vet to flush the system. He should see a Herp Vet for a complete check up. If you supply a location, I can find some Herp Vets for you.
In the mean time only Plain Calcium and also keep a dish of plain calcium in the tanks. We can try to hydrate the Gecko by giving a soak in a 50/50 warm water and Pedialyte soak. You will need to support the gecko since he is twisting and turning. For feeding you can add the powder that what you are feeding to some chicken or beef baby food and drop on the snout, and allow the gecko to lick it off.
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