personally i like the chocolate tangs, but in your tank i would do a purple tang.... they stay quite a bit smaller, and seem to be a little hardier.... but keep in mind add any fish that you plan to add before the purple, cuz they can be very boligerant to new tank mates of any species.... my purple pinned my yellow tail blue for the first half hour of its life in my tank, but i put 2 nets in the tank dividing it, and now they are just fine...
shane
