Call the Vet and ask him what would be the correct dosage. He might want to know how much the turtle weighs, so he can prescribe the correct amount to give to the turtle.
I think you need to get on the phone and call your vet up. Ask him to tell you because when it comes to your pet only take the word of the vet that saw him.
If you have already spoken to your vet about this issue you should be able to call them and they could tell you the correct dosage. These types of antibiotics that are prescribed by a vet need to be given in the correct dosage. While you turtle is recovering make sure to keep his basking area nice and warm.
