I had some Hynobius for a while and found it hard to come up with a setup that worked well. They like to be in shallow wet hidden spots. I ended up with a tank with water, a 12x12 inch piece of slate, and a pile of cork bark on top of the slate. The water level was kept so that there was just a bit of water above the slate. This setup was easy to clean, and it made the sals happy, but it was not the most attractive.
Did you see the setup photos on the H. dunni caresheet page on Caudata Culture?