Setting up a Clown Fish tank

They are great fish. You don't need a reef for Spine-Cheeks, years ago as a teenager I kept them for years using simple undergravel filters and a little live rock. But my smallest display was a 29; a 20 gal is kinda small for the Maroon; they are one of the larger clowns and mature at 5-7 inches so a 40 gal would be better. They can get scrappy so best to start with one small specimen not a group.
Here's more info;
http://www.wetwebmedia.com/maroonclnart.htm

A percula would fit the tank a lot better IMO. They don't get as big and they would get along well if you wanted to add another small tank mate."not a different species of clown or a damsel." Just make sure that everyone has a place to hide.

I agree, I wouldn't do a Maroon in a 20g for its entire life. Like FloridaBoy said they grow too large. I too would go with a Percula or an Ocellaris, Pink Skunk, or one of the other smaller clowns. I would say the 2 to stay away from (as far as size goes) is the Maroon, Tomato and the Clarkii. Some live rock will work fine for them. Most all clowns are pretty bold and will spend the majority of the time in the open. The one thing to stay away from unless you invest in very expensive lights (Power Compacts or Halides) is the common clownfish mate of the Anemone. They require lots of lights and if you buy a captive raised fish most cb clowns won't even host in anemones. Anyway, hope this helps and good luck!