Hydrocotyle tripartita will grow well in lower light with no co2. The trick is to keep grooming it and steering it in the right direction. it has the tendency to grow up into a ratty bush, but if you keep pushing it into the substrate it'll eventually just take off like a runner. You can cut parts off of it and replant at will. It won't ever carpet like HC, but it'll give some good cover fairly quickly. Mine ran 1/3 the length of a 20g long, turned the corner and is halfway to the back wall, all within about a month.