I think it depends on the variety of clarkii. Some stay relatively small, so a pair of these would be okay in a 30 gallon, provided there weren't many, if any, other fish. The larger clarkii's, I would not keep in a 30. Members of the clarkii complex are the most mobile of all clowns, IMHO. Their natural territory is much larger than other species, so I believe they need larger tanks to live in. I have kept a very large pair of maroons in a 30 gallon, and they only used half of it. A clarkii in the same sized tank, will be all over it.