Right now I am using a ReefBreeder 24 Photon V-2+ and my tank is also 30" across, which is the same as your 20 gallon Long. It might be more investment than you are wanting now. It's what I would describe as the more reasonably priced of the nicer LEDs. It has 6 different colors so you can tailor it for whatever you have.
It will grow anything you want, from macroalgae to SPS coral. Here is a pic of my tank. You can see codium, cheato, halimenia, ulva and red grape caulerpa. You cannot see opuntia and caulerpa cupressoides but I have those too.