A quarantine tank is a separate tank you can set up to separate fish from your main tank.
The tank usually only contains a heater and a filter, and no decorations, bare bottom too, meaning no substrate at the bottom. This is mainly for low maintenance, less risk of a sick fish injuring itself on decorations, and an open space allowing for observation. Having the light turned off can also reduce stress, only turning on when observing.
People usually use a smaller, internal filter where they can quickly get to it and clean it regularly. Something powerful but small such as the Fluval U series are usually a good choice. The Fluval filters are very versatile and great quality builds.
Quarantine new fish
One of the main reasons people do this is to quarantine new fish after purchase. New fish from the pet store can contain harmful diseases and infections that could potentially spread to all your existing fish. In some cases wipe out the tank.
A quarantine tank is set up so you can keep these new fish under careful observation without causing any risk to your existing fish. The duration in which fish are kept in quarantine can vary. About 3-6 weeks is a popular option.
Another reason people set up these tanks is encase one of their existing fish because infected or develops a diseases. You can then separate that fish from the rest, to stop it spreading. This is usually called a hospital tank.
A separate tank also allows you to medicate your sick fish. Without it effecting any other healthy fish that don’t need treatment. It’s always best to not treat fish unless necessary, you don’t want to add chemicals unless you have to.
Raising fish fry
The last reason to have a quarantine tank set up is so you can raise fish fry, away from other fish and in some cases their parents, depending on which species. Some species of fish will actually eat their own fry, so separating can ensure a much higher survival rate. Also target feeding.
When feeding fish fry, larger fish can easily outcompete smaller fish with them for food, and the fish fry won’t get a look in. If they are in their own tank then this won’t be a problem and you can target feed knowing that they all get a chance.
The only negative
The only downside to a quarantine tank is obviously space. Having an extra tank running isn’t an option for some people.
However when it comes to fish fry, some people use a hatchery. A small plastic, or mesh netted box that sits inside the main tank. Secluding the fry, without the need for an extra tank. This way they can still be kept safe from larger fish and the intake pipe of the filter. You can also target feed them.