I've heard there is a mod you have to do for it to work on your 20g. The nanocube has a greater depth front to back (20" ?), versus your 20g is 12". You take apart the mount and "reverse it". Can't remember where I saw it, prolly on nanoreef
I have my coralife clamp on 150w hqi on a 20g temporarily (still working on modding my nanocube
, and its not *too* far forward, but it could be better.