It's virtually impossible to achieve a satisfactory temperature gradient in a 20 or 30 gallon tank, you will need something larger. I personally would say 48" at a minumum. Uro's diets vary greatly but consist mostly of nutrient dense greens. Most offer food once a day and supplement every few days. Ambient temps = 100-80 basking 130-110. Low humidity. They do not need standing water, they hydrate from the water in their food. Most consider plants not to be a good idea, but assuming you got a large enough tank and had the proper soil, creosote, chicory, or dandelions could be grown in the tank, but it's still not very practical due to how long it takes to grow them, the amount of water they need and the adverse effect it would have on the Uro as well as how much an adult Uro could eat in one day. If you have any other questions feel free to ask and try to learn as much as possible before getting the Uro. Hope this helps 8-)
HH