I ordered the 18” for my nuvo 10 and found it to be a little too long for my preference. In order to get the light 7” off the water like I wanted I had to clip it on one of the very back corners and bend it into a bit of a hump. 12” is a good option for a cleaner look, but 18” is a better option if you have a tall skimmer in the middle chamber or something else that blocks easy placement of the mount on the middle.