A Quarantine Tank is a separate tank which every aquarium hobbyist should have. A Quarantine tank helps isolate new fish which have been brought recently. New fish can carry disease-causing microbes or parasites which can decimate your aquarium.
Hence, each new fish should be kept in observation for at least a week. Many hobbyists keep their fish for about 4 weeks as some disease take longer to manifest. If the fish are healthy after the quarantine period they are introduced into the tank.
If any fish die during this period, the water should be changed and the quarantine period for the other fish should be extended.
It is unfortunate that many aquarists neglect this important precaution and regret it later. The quarantine tank can also double as a hospital tank or a breeder tank for fry depending on the availability.
Any tank which is above 10 gallons can serve as a quarantine tank. Even an ordinary bucket can serve the purpose. Just add an air supply with an air stone.