I don't want to sound harsh but the enclosure setup is unsuitable probably causing the stress.
The size is great but stones are a BIG NO as they can be swallowed causing impaction then death.
You will need to replace the substrate for organic soil (no fertilisers), this will greatly reduce the chance of impaction. For water you could use a water bowl or use a perspex/glass strip and use aquarium safe silicone to seperate the land and water areas.
Live plants are best but fake are fine if you want to use live plants I suggest bird's nest ferns or brazilian ferns as they are hardy plants.
You will need branches, logs etc (boiled or cooked in the oven to remove any nasties) as Bell frogs love climbing and hiding under objects.