I agree with BlackAngel, at 4-5inches long he should be upgraded to a 20gal+ (although the length and width of the tank is most important- there's no point in getting a tall and narrow 20gal as it will be the length/width of the tank that is the most important to the goldfish). Of course if you can something even larger than a 20gal, that will be even better . Black moors usually only grow to 8inches tops on average.