You can put in a Snowflake, but it's gonna be a hassle of finding someone to take it when it gets bigger. A fish store will tell you, now..."sure, we'll take it back" but what are you gonna do if the time comes and they change their mind? It's always a good idea, when getting fish, to make sure you have proper room for them, all the way up to their adult size.
I have a 12gal JBJ Nano Cube and I keep 2 Anglers in it...you can check out my "New Frogs" thread, in the aggressive fish forum.
As someone else said, Leaf Fish would work, small Scorpionfish...groupers and Morays just get too big, too quickly, to put in a 12gal.