The origin of the word Quarantine stretches back to the Middle Ages.
The practice of quarantine, as we know it, began during the 14th century in an effort to protect coastal cities from plague
epidemics. Ships arriving in Venice from infected ports were required to sit at anchor for 40 days before landing. This practice,
called quarantine, was derived from the Italian words quaranta giorni which mean 40 days. ~~ Center for Disease Control, USA
In the Aquarium hobby, Quarantine means that any new additions to your tank or tanks must go through a process to ensure that the new arrivals are not sick. This will require a cycled aquarium, usually 10 gallons as most medication dosages are in 10 gallon increments, a lid, and a heater if the fish require additional heat.
Some hobbyists simple observe the fish, looking for symptoms of common diseases. Others simply treat the new fish for the most common problems in the species. Since I keep Guppies who are notorious for having Ick, Fungus, and Internal Parasite, I treat for those conditions in my Quarantine Procedure.
This is the procedure that I now use. It is not my original method, I learned this from Cory of Aquarium Co Op and have been using it ever since to excellent effect.
In a fully cycled, heated, and biologically filtered aquarium; Dose:
API General Cure
API E.M. Erythromycin
Hikari ICK-X
Acclimate your new fish and transfer the FISH to the Quarantine tank and not the BAG WATER! Keep the possible disease load as low as you can by ONLY transferring the fish.
Watch the fish for a week for signs of disease or stress. I only feed during this time if they will eat and I feed a very small amount so as not to overload the filter. Do a 50% water change after the week is up and again observe the fish. If no disease is present you can move them to their final home. If you see any signs of disease at this point you need to decide if further treatment is warranted, some fish never make it out of Quarantine.