I have a 16 gal bowfront with the 20" orbit fixture. Thriving inhabitants include:
six line wrasse
percula clown
30+ various snails
12 hermits (blue and red)
Xenia elongata (that I am having to frag at least every two weeks)
pom pom xenia - started with two now have 7 in 3 months
couple blue and red discosoma
5-6 ricordea that I have fragged and regrew
3 colts (two are frags)
many and various zoanthid colonies
gold crown sarcophyton
hairbrush sarcophyton x2
red finger gorgonian
3-4 colonies of green stars
coralline algae everywhere
So, IMO the orbit fixture is plenty of light unless you want some SPS that may need higher light. Go to Reef Central and look under 'All things Salty with Anthony Calfo' and check out his nano...18W of PC and he has SPS!