Generally, I find if you have a tank with at a bare minimum of 50 pounds of porous rock, and a 2" sand bed, you can sustain 1 mandarin as long as there are no other fish that eat copepods in the tank. Things like six-line wrasses would compete for food. The bigger the tank and the more rock, the better the chances of success are. It all depends on your system. Mandarins will also eat things other than pods. I had one in a tank that had a couple large stomatella snails in it. After I moved the mandarin to another tank I had literally 100 baby stomatella shortly thereafter. The mandarin had been eating the baby snails.