Thank you, seaweed. I ended up getting an Aqueon top with the light already in it, after searching for hours online, and afraid I'd get one that wouldn't fit. Since this is an Aqueon tank, at least I can be sure it fits. I love the Versa tops, and have them on my 55, but I had trouble finding a light that was 30" for the 20L. That was my main problem. I should be getting it soon. They shipped it yesterday.
My favorite fish are female bettas, Balloon Mollies and Peacock Gudgeons.