Just to tag onto Lynn's excellent response, I have used the reptifogger and its nice. It helps increase humidity which can never be too high for darts unless they're swimming. It also looks pretty cool to show off for guests. A 20 gallon high is fine for a pair to 3 Leucomelas. Provide ledges wood and plants for them to perch from and your viv will be very suitable for them.

The best substrate to grow plants In my not so humble opinion is ABG. Many frog retailers carry it and it usually goes from 5-7 a gallon. For a 20 H vivarium you'll need approximately 3 gallons to cover 1.5 -2 inches. If you want to id be happy to share the name of the retailer i use for most of my supplies but in private so as not to upset any sponsors. Best of luck and take lots of pictures.