Most of us use heat tape or heat pads, usually under the viv, to provide our heat source. Corns don't require light, beyond normal daylight, but some people like to have a light to watch their corns. I don't know if that is even necessary, as most corns stay in their hides.
30c is okay for the warm side, but 20c is too cool. I wouldn't go below 23 or 24c.