Well, the sides are a bit of an issue as the 55w kit just isn't wide enough for good coverage on the 20g long. If I plant stem plants on the sides, then they grow diagonal to the center.
Front to back is mainly the issue. These fixtures are good, but if not that high over the tank, then most of the light is focused in a narrow "band" down the center of the tank, IME. This is also due to the 20g long being quite shallow.