My recommendation for aquarium size is just that - a recommendation based on my experience keeping these animals, what I know about their behavior in the field, physiological tolerances and other factors such as their tendency to bury food and molt skins. If you want, you can keep an adult N. wennerae in half a liter of water changed every four days (I have about 50 being held in the lab just that way.) However, if you want an animal that grows at a normal rate, reproduces and behaves as they do in the field, then a stable, larger tank is required.
N. wennerae/bredini has been my primary research animal since I first worked on it in Bermuda 40 years ago. I have collected probably close to 100,000 of them, spent thousands of hours watching their behavior and have hundreds of hours of video taken in the field. While when maintained in a small aquarium and provided food at or near the cavity, they may remain secretive. In the field where food (and mates) are more scarce and must be searched for, adult N. wennerae make several ftrips away from the cavity starting just after dawn, slowing at mid-day and picking up towards sunset. Cavities are always closed by dusk and opened at dawn.
Actually, behavior in an aquarium doesn't really look much like field activity until the animals are housed in a really large tank, but a 10 gal is a nice compromise that promotes good water quality and gives the animal room to move about.
Can they be kept in a smaller system? Sure. Will they behave as they would in the field? Probably not. Will they they grow and reproduce as much as in the field and live as long? No. Are they as happy? I have no idea.
