My 10g is also about 20in and I have the 18in Ray 2, but the legs seem very unsteady pulled out enough to fit. Fortunately I have a cover that the light sits on. I would not feel comfortable putting the 18in on my tank without the cover, the legs are just too wobbly pulled out to fit 20in.
I do however think that tthe 18in light is the right size, the legs were just poorly designed. If I had no cover, I would probably still get the 18in and just figure out a modification for the legs.
Sent from my Transformer Prime TF201 using Tapatalk 2