Mandarins are problematic for a lot of people because we don't pay enough attention to the natural diets of fish. When stocking a Mandarin you must assume that he will only eat zooplankton in your tank. If you've got a very large well-established ecosystem with many large fishes, then the mandarin has no competition for the copious amounts of zooplankton he gets.
When you add the mandarin to a 20-30 gallon and try to make up for it's lost natural food, there are quite a few problems that can occur. The first is that they don't take anything prepared, including cyclops, which is most common. The aquarist then decides to put in a bottle of zooplankton to boost the population, or maybe buys a bottle and feeds it regularly to the tank. If your tank is 20-30 gallons, pretty much every single fish appropriate-size for the tank eats almost entirely zooplankton in the wild. Dumping in this product just acts as a buffet for all your fish, with the slow mandarin getting the leftovers, and the zooplankton never reproduce in your tank.
The fact is that the zooplankton actually require feeding to breed and survive in the aquarium is another common reason for failure. Some tanks can get away without feeding phytoplankton to their copepods. Many modern tanks are using filtration that removes the greenwater from the water column, and it is essential that these are turned off during feedings.
As I breed my own zooplankton and have large quantities, I am able to keep mandarins in tanks of any size, with as many other pod-eating fish as I'd like. My 20 gallon, 10 gallon and 6 gallon all have/had Mandarins in them, two of them are going on 6-8 months, the other is over a year old.
I set the tanks up as an active culture, understand that all the fish will eat the pods, and feed the tank nothing but phytoplankton. It is almost totally self-sustaining in regards to feeding, and could be totally self-sustaining if I sat it next to a window and fed live phytoplankton that would multiply.
