Every time a hobbyist adds a new animal to his display, there exists the risk of introducing a diseased fish which can them infect the rest of the aquarium population. Most hobbyists are not trained to recognize key symptoms of a diseased or ill fish. Experienced hobbyists know to look for fish that are active and not lethargic, eating well, have clear eyes and fins, and no spots on the body of the fish. Some symptoms may be very subtle and difficult to recognize.
Traditionally, most retailers purchase fish from a local wholesale operation. These fish are then brought to the store and sold to the first hobbyist with the money. These same retailers will typically tell the hobbyist to purchase an additional tank to be used as a quarantine tank. However, if the hobbyist cannot diagnose a sick fish, how successful will that hobbyist be when trying to appropriately medicate a quarantine tank? What are the different symptoms and treatment of a fluke infection vs. a bacterial infection vs. a protozoan parasite?
The most effective way minimize the introduction of disease to your show display is to purchase fish that have been quarantined for a minimum of 14 days and medicated appropriately by a trained professional. If the retailer is not willing to quarantine, then that responsibility falls on the shoulders of the hobbyist.