I have used a half dozen nano skimmers for my fusion 20 over the years and my favorite would probably be the Icecap k1 Nano. Decent design and pretty good skim considering the small footprint. It is basically a smaller version of an in-sump style skimmer.
The pump is questionable though and I actually broke mine yesterday when trying to take it apart for cleaning. Also, the included air silencer is useless and you will probably want to replace it with something better.