Honestly I would not put a clownfish in that size tank. Yes they may not grow to full size but that's not a good thing, that's unhealthy and stunting. Find a couple full grown clownfish, it's amazing how large they really can get. I also don't think a 12 gallon should be a folwr though, if you want a fish tank buy a larger setup, somehow nano REEFS have become nano-shove as many fish as possible tanks. I would go for the high fin goby and cover the tank, buy a couple small cool inverts for something to watch. Watchmen gobies get to 4 inches, again not a good choice for this size tank. Sixlines are too large and active, court jesters often do poorly in small tanks.