The problem isn't just how much they can eat, it is what they can eat. Many mandarins are very picky and will only eat live food, which can only be supplied by a tank with a large pod population, so usually a large well established tank with a refugium will meet this requirment. If you can find one that will eat frozen or prepared food then you would probably be able to keep it, but it will be hard finding one that will do so.