Since hermit crabs are marine dwellers in their native habitat, it is essential they have supplies of salt water. Even if this is provided in shallow dishes within the tank substrate. In the wild, hermit crabs scavenge dead fish or decaying food waste. Calcium is essential in their diet to grow their skeletons.
With abundant marine plants and fallen tree leaves, foliage, fallen fruit and vegetables they are mostly herbivores but they consume decaying carcasses with relish so they are in fact omnivores. However, they require acidic tank water with PH level 6 being considered as ideal. In your aquarium, pieces of carrot and cauliflower and cabbage will be easily consumed.