So, you''ve cycled the tank, got fish and want more. You check ammonia and nitrite and everything is fine. Off you go to the LFS (Local Fish Shop) and get your new additions. You follow the instructions to acclimate the fish and set them free into their new homes. Within days, maybe a week or two, your fish are sick or dying. Both new and old. You check ammonia and nitrite again, and all is fine. So what went wrong? The good news is, it's not your fault the fish are sick. They were likely sick when you got them. The bad news is, now there's every chance your entire tank is infected. Some, like whitespot and fungal infections are easily treated. Others like columnaris are much more serious, harder to treat (especially if you're in the UK), or worse, impossible without antibiotics, and potentially lethal. Columnaris in particular can strike quickly and wipe out an entire tank within days, so quick that the only way you know something's wrong is when the fish start dropping like flies. A Quarantine Tank (QT) doesn't have to be an actual tank - it can be as simple as a food storage box, but it does have some requirements it has to meet. It will need to be big enough to comfortably house the fish for the duration of their quarantine period. For a shoal of tetras, that might something as small as a 20ltr box or tank. For larger fish, it may need to be 70ltrs. My current QT In addition to a tank or container large enough to house the fish, the quarantine tank will need; 1) A filter. You could use a sponge filter and keep it running in the main tank between uses, but personally, I'd recommend a gentle internal filter such as the Eheim Aquaball range. That way you can keep the actual filter in the QT between uses, and simply take some filter media (sponge, bioballs, ceramic rings, etc) from the main filter and use it in the QT filter. 2) A heater, if the fish are temperate or tropical. 3) Decorations and places for the fish to hide and feel secure. 4) Separate gravel cleaner and buckets for water changes - ALWAYS keep water changing equipment separate - especially that of your main and quarantine tanks. Using the same equipment for the main and QT tanks means running the risk of cross-infection, which is the whole point of quarantining new stock. 5) A hood or lid - especially if the fish species are known jumpers. A thin layer of sand or gravel will also help the fish feel secure, but isn't strictly necessary and it is easier to keep the bottom of the tank clean without it. Likewise, unless there are real plants in the tank, lights aren't important. Duration Quarantine should last a minimum of 2 weeks, ideally 4 or more. After that, you can be sure that some of the more virulent and common diseases that can affect fish will not be passed on by the new stock. While the QT is in use, it's important not to add any additional stock. In the event of a disease breakout, adding more fish will mean you can't be sure which of the additions brought the disease in with them. Once the fish have gone to their permanent tank, thoroughly clean the QT and store it until you next need it. Other Uses It's not just the quarantine of new stock that a spare tank will come in useful. Other reasons include: Hospital Tank: Isolating a sick fish for the purpose of treatment. Breeding Tank: Pretty much as it says on the tin - separating fish for the purposes of breeding them, which leads to... Nursery Tank: Whilst there are a few exceptions (particularly the Anabantids and Cichlids), most fish aren't great parents. They spawn, and then turn right around and eat their own eggs and fry. If you wish to try your hand at raising fry, you'll need a separate tank in order to protect them from the parents. Conclusion Using a spare tank as a quarantine tank is worth its weight in solid gold. It can spare you from the soul-destroying experience of watching a once thriving tank slowly decline and die, allow you to catch diseases early, isolate the sick for treatment, and give you the option of breeding the fish and raising their fry. They don't have to be complex, and they don't have to be run constantly. I hope you've found this post useful. Happy fishkeeping.