20 is definitely too small. Not meant as a rude comment, but, unfortunately, a lot of people see any
clown & think "Little
Nemo", not realizing that some like Maroons & Clarkii's can get upwards of 5" to 7"...that's a big fish !! And clowns are very thick.... think of a maroon clown like a small SW
Live Aquaria recommends a minimum
of 30 gallons, but I think a 75 to 90 would be much more comfortable for something that gets that size.