That's a small terrarium, even for dart frogs, particularly for beginner species, which I assume you would get since you've never kept darts before? Such a small terrarium is hard to keep "natural" because you will have difficulty maintaining substrate, live plants, moss, etc, and lighting will be challenging (I don't know of any commonly available hoods for 5 US gallon aquariums that come with fluorescent bulbs, so things will get hot with the included incandescent bulb(s)).