I've owned the 20.5" on a 24" tank and now have the 27" on a 36" tank. In both cases I found the light spread to be quite poor when kept leg mounted - hot spots and shadowed corners. Suspending the fixture around 12" from the water surface allows much better dispersion aesthetically and practically. If you do suspend the fixture, I'm quite sure the 120W will work. It is also worth noting that the 3 LED arrays on the 27" fixture fall within the center 16" which could be quite nice. If I was shopping second hand, I'd simply pick whatever is available.
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