Quarantine tanks are important for protecting your current fish against infections potentially carried by any new stock you bring into the tank. Fishlore has seen more than a few instances where a tank was entirely depopulated by the addition of one fish.
A quarantine tank isolates new fish for several weeks or a couple of months, providing them with a chance to show signs of infection and/or recover from an infection, reducing the possibility of infecting your tank at large.
Here are a few links on how to set up a quarantine tank:
http://www.fishlore.com/QuarantineTankSetup.htm How to set up a Quarantine tank Quick and dirty quarantine setup
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