If your crested gecko is losing weight, try changing the flavor of prepared diet and offering more insects (wax worms are great for encouraging weight gain). If the crestie shares a tank, separate them from their roommate(s). Finally, call an exotic vet — impaction and parasites require medical attention.
Using a kitchen scale to weigh your gecko weekly is an easy way to stay on top of your gecko’s health. Rapid, unexplained weight loss can signal a deeper problem.
Skinny crested gecko. Photo credit: CGMa on the Pangea Forums.