Well - as your living in Southern California, I would suggest maybe the Gray Smoothhound. Smoothhounds tend to do better in smaller ponds than active ORVs like the Atlantic Sharpnose. A young Smoothhound or Japanese Banded Hound could likely stay in a 8 ft diameter pond at least for the 1st year to 18 months. Even as a Adult most Smoothhounds could be keep in a modest size above ground(15-16ft in diameter).
As Japanese Banded Hounds - since they can grow to be about 5 ft long, would likely need at least a 20 ft diameter pool, as adults.
It should be noted - that the so called "Smooth Dogfish" is a fisherman's name for a Smooth Hound Shark. Which is not be confused with the Dogfish. Dogfish are cool water sharks - often found in water with temperatures of 40-55 F. Smooth hounds are closed related to Leopard Sharks and th Japanese Banded Hound Shark - as such the seem to prefer water temps - usually about 55-72F.
As for basic care - Smooth hounds tend to be a bit easier than active ORVs like Sharpnose. But they still aren't as easy as Bamboos, Eppies, catsharks or Horn Sharks.
