Bigger is better. I am in the process of redoing my darts in either 33x17x20 (about 50 gallons) or 45x17x20 (65 gallons) custom tanks. Having a larger enclosure allows for more options in hardscaping and planting, increased surface for microfauna to colonize, and the creation of more stable microhabitats (areas of higher or lower temperature, humidity, light, etc.) than a small enclosure.