A quarantine tank is just want it sounds like- a separate tank designated for the quarantine of sick or potentially sick fish. It serves two main purposes- to provide a safe and controlled place to administer treatments and to prevent disease from reaching your main tank in the first place by providing a location for new arrivals to be checked out. Many medications and treatments are simply unsafe for use in the main tank- they may kill all invertebrates, plants, and algae; wipe out the biological filtration; be difficult to dose correctly; or be difficult to remove when the treatment is finished. Furthermore, medications generally aren’t something you want to be exposing your fish to unless it’s necessary. Having a quarantine tank allows you to treat only the fish that need it and only for as long as is needed. In any case, a separate quarantine tank is necessary to successfully treat your fish without making things worse in the end.
Setting up a complete second tank for quarantine is a bit of a hassle, but it doesn’t have to be as bad as it sounds. It doesn’t have to be pretty nor does it even need to be an aquarium- a sturdy plastic tote can do in a pinch. A quarantine tank is at its best when it’s just the bare essentials- namely filtration, a heater, and maybe some additional circulation pumps. None of this needs to be anything fancy. A simple HOB filter will work fine (remember to leave any carbon out as it will soak up most medications) as will some cheap basic powerheads. The cheaper the better in fact when it comes to the setup for some treatments, namely copper. Some copper based medications are quite pesky when it comes to removal and will contaminate the tank and everything used in it making them unsuitable for use in the main aquarium ever again. As for other things you will need, a light is nice for checking the progress of the treatment but isn’t essential. The common test kits are important as well as even if a proper biological filter is in place some medications can disrupt it and so it’s best to keep an eye on things yourself. Separate tools such as nets, brushes, and anything else you may need to maintenance are important too to prevent inadvertently transferring disease back to the main tank. Finally, the tank itself should be kept more of less bare with only a few short lengths of PVC pipe to offer the fish a few hiding places. Substrate isn’t required and usually just ends up getting in the way during maintenance.
A quarantine tank can be run continually in case the need arises, but for most people that’s not a good option. Luckily, one can be thrown together relatively quickly if the necessary equipment is kept on hand. As always, stable consistent water is key, so do a water change in the main tank at the same time and use the old water to fill the quarantine tank. Don’t make this a habit, however. Once the tank is up and running and treatment has begun never move water between the two tanks. The other part of keeping that water stable is of course the biological filtration which, with the tank being new, wont exist. As a consequence, ammonia will begin to build rather quickly. This can be combated with water changes and by keeping a spare bit of filter media for just this purpose in your main tank that can be moved over. It won’t fully establish the necessary bacteria but it will jump start the process. Just remember to toss the media when quarantine is over.
The amount of time fish may need to stay in quarantine can vary greatly depending on what the problem is and how treatment is going. For new arrivals a few weeks is usually sufficient time to observe them for any potential problems. On the other had, some diseases can take months to eradicate completely. But in either case the point is that your fish may be in this tank for a long time so apply all the rules you would when setting up any other tank. Make sure it’s large enough, of course, and ensure it’s in a location that won’t be problematic.
It’s rare to see a tank with but one solitary inhabitant. Most aquarists eventually end up with a community aquarium full of all sorts of different fish. And all it takes is one slip up for disease to decimate it. Quarantining each and every new addition is a bit of a nuisance for sure, but well worth it. There’s a long line of aquarists who wish they had done just that.