Many of the common fish health issues aquarist encounter can be prevented by providing the correct water chemistry, and a good aquarium set up for the species of fish they will be keeping. Fish that are kept in closed environments tend to pass diseases quickly do to small water volume and heavy fish populations. Parasites do not have to go far to find a host in an aquarium ecosystem. In the wild, large volumes of flowing water makes it difficult for parasites to reach epidemic proportions.
Aquarium fish are most often hatchery raised with some still collected from the wild. Fish are most often staged in a wholesale facility before they are shipped to other wholesalers throughout the world. Fish are often only kept at a wholesale facility for a few days before being shipped. While at wholesale facilities, fish are kept in high stocking densities that allow for the easy transmission of disease. Fish that are infected in these facilities are then sent to other wholesalers, thus infecting new fish. Most wholesale facilities do not quarantine new fish as this would be cost prohibitive.
Local wholesalers ship new fish the day after they have been received to your local fish stores. It is the local fish store that normally ends up treating newly arrived fish for diseases the fish picked up in transit. If you bring home fish from your local fish store that have been there for less than two weeks, your risk for bringing a disease home is greatly increased.
Typically the safest place acquire new fish is directly from a breeder. Buying breeder direct allows you to avoid the primary source of disease transmission, the wholesale chain.
The best way to prevent the spread of disease is to quarantine all new fish. A quarantine aquarium can be as small as 10 or 20 gallons, depending on the size of fish. It should be a cycled aquarium, with water chemistry similar to the water chemistry of the aquarium they are coming from. It is best if the quarantine aquarium has a tight lid on it to prevent any possibly contaminated water from escaping. The quarantine aquarium should not be near any display aquariums, and it is best if it is in another room or building.
Any nets, test kit vials, or other equipment should not be shared between the quarantine and display aquariums. When feeding fish, feed the quarantine tank last. When working in and around the quarantine tank make sure you wash your hands thoroughly when done.