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Are you a beginner? Green Chromis is a nice beginner for a small tank. The are pretty even tempered, you can find them tanks raised and they are pretty hearty.
I'm a freshwater guy. I'm sure the Salties will chime in soon.
For best care, feed several times throughout the day with a varied diet of meaty foods, herbivore flakes, and frozen preparations. Vitamin-enriched foods may also help to preserve body colorations.
BTW a 20 gal. is pretty small for salt water. It's not imposable, just harder for a beginner. I would recommend doing some research if you haven't already to make sure everything goes well.