Since you want to keep it in the basement, a light would absolutely be neccessary, unless you have a basement with unusually large windows! There are two excellent old threads on this forum answering your questions. Do a search of this forum on 'Closed-System Terrariums' and 'Plants for closed terrariums'. Or just search for 'DianaMay" (the person who wrote these threads). Even if you don't want to keep yours closed all the time, read the threads. I think we really need an FAQ for this forum. . . people are always asking the same questions! =) The temps. shouldn't be an issue, as the light will put out some heat. Just make sure that it's not drastically below room temperature at the same time as being very humid, or the plants will rot, unless you are keeping plants from the cloud rainforests, like certain orchids. But you probably don't want to get into that. =) a 20 gallon tank is a beautiful tank to start out with. If you have a screen lid, replace it with a glass or plexi lid. Screen does not keep any humidity in, and one of the main points of growing plants in terrariums is the extra humidity. For lights, use fluorescents or compact fluorescents in a Daylight or Cool white spectrum. Search this forum for 'Lights' or any of the main words in the previous sentence, and you will come upon much info., including how to make your own light fixture if you are handy. So read old threads on this forum, and then come back with more specific questions, okay? And welcome to the addictive world of terrariums. =)