Substrate Hazards

Editor's note- Dr. Rosskopf and myself have both seen intestinal impactions and rectal prolapse in geckos housed on sand or fine gravel yet vermiculite seems to pass right through them. For tortoise hatchlings I use newspaper, alfalfa pellets, and medium bark chips. Indoor!outdoor carpeting also works well but make sure the tortoises don't eat the frayed edges. Indoor outdoor carpeting frays less than artificial turf. Of these substrates I prefer alfalfa pellets although they can get dusty and mold if moist. The pellets should be changed frequently. Recently, I saw several (3 out of 4) leopard geckos, (Eublepharis macularis) suffering from weight loss and anorexia that had severe gastrointestinal impaction from crushed walnut shells. Has anyone else seen problems with crushed walnut shells?