If you don't want anything bigger than a 10 gallon select a fish that will live a happy healthy life in it. A 10 gallon is much to small for a clown and if you say am going to upgrade why waste money on a small tank now. A 30 gallon tank is actually more stable than a 10 gallon and much better suited for a clownfish or pair of clownfish. BTW I know of a few people whose clowns have lived over 20 years. Fake coral is fine no problem there.