Most tangs need a 6' tank around 180 gallons. The Ocellaris Clownfish will be good for a 20 gallon. I use this site to research anything before i buy. Nano Fish
Check the Nano area. I have a pair of Ochrestriped Cardinal fish rated for a 55 gallon and I'm looking to add some Yellowstriped Cardinalfish their rated for 10 gallon minimum. Good luck!