Not recommended. Clowns will outgrow a 20, but slowly. The bigger issue is that they are very social, and like to be in groups of at least 3 and preferably more. So, the slow growth is offset by the numbers. You could have a trio of small ones in a 20 for about a year, if there were very few other fish in there. After that, you'll need to upgrade to keep them happy. And, clowns tend to be prone to diseases when not kept happy, so this is really important.