29 gal should be fine for one pair. most recomend a tank that is 3x the length of adult horse. kuda 4 to 7 inch and erectus 5 to 8 inch. Mustangs ?? kuda or erectus. Do you plan on breeding?? if not then get two females. try not to pack it to tight with live rock, leave some open space for swimming and exploring with plenty of hitches.