I would say its time for the vet to check him out. Unless I missed something it seems your enclosure is fine. The fact that he spends a lot of time in the water bowl concerns me. Twenty-four hours before he died, my "large" blue poison dart frog sat in his water bowl and didn't leave it. Before that he ate a large amount of been weevils, which are high in chitin, a substance known to cause impaction in the gastrointestinal track. Has your frog been eating mealworms or superworms? These are also high in chitin, and feeding them too much or exclusively can lead to problems.