Originally Posted by Age of Aquariums
Clowns (if I remember right) need 20 gallons. But if you're on top of the water conditions, then you can get away with one ocellaris clownfish. I'd also look into neon gobies.
Clowns can be put into 10G, but they do much better in pairs in 20G. It can be hard in a 10G, but it can work.
The 12G Nano cube is about 8-10 gallons of water. Gobies are still really good here. Maybe a shrimp goby/pistol shrimp pair? Those are fun to watch. There are many that can fit in here; Yasha Hashe, Yellow Watchman...the list goes on.
If you wait a few months, then you *may* be able to add another fish if you water parameters are all fine and have been for the past few months. Only if your tank has been running perfect for a few months.