Any good fish professional worth their fins should always recommend that all fish should be quarantined before being introduced to a new environment. Whether quarantine can be practiced or not is another issue. The process of quarantining fish is generally agreed upon by most professionals, the length of time in quarantine is still debated among fish professionals and veterinarians. Quarantine defined is: Isolation typically to contain the spread of something considered dangerous, often but not always disease. The word comes from the Italian language, quarentena, meaning 40 day period. The objective of quarantine is to assure the health of the fish by allowing enough time for any health issues to develop in the fish before entering into the general population.The process is just that, isolation. The isolation usually taking place in what is referred to as a hospital or quarantine tank, before introduction to the general fish population. During quarantine the fish should be observed very closely to assure that: there are no signs of stress in the fish, there are no exterior signs of parasitic, bacterial, or fungal infection, no signs of damage to the body of the fish, and that the behavior of the fish is appropriate for the species being observed. During quarantine the fish should be fed regularly with medicated and non-medicated foods, small frequent water changes should be performed, and at the end of the quarantine period that you feel comfortable with, the fish should be acclimated to its permanent home and again be observed for at least the first few days to make sure the fish adjusts to its new home.The length of time in quarantine is debatable and somewhat at the discretion of the fish owner. Anyone would agree that quarantine should take place for at least 15 days, others have said that quarantine should take place for a period of 6 months or more, so; as can be seen this is a great difference of opinion. My opinion is that 15 days is too short, especially knowing that the common ich parasite can be infesting a fish and not show any signs for up to 17 days, other problems can develop at a slow rate as well. I also think 6 months, for most people, is too long unless you are a collector with some expensive specimens. I do think a very reasonable quarantine period is 4-6 weeks. A 4-6 week quarantine will allow most problems (parasites, bacteria, fungus) to reveal themselves and present the opportunity for treatment if need be, and this period falls within the classic definition of quarentena.