When Would I Need To Get A Quarantine Tank?

It's all up to how cautious you are.

The tank's maturity has nothing to do with the condition of the fish you buy from a pet shop. It doesn't matter if your tank's been up and running for a month or a decade, if you add some new fish carrying a disease, there's good odds it will infect the other fish too.

Sometimes fish look fit and healthy in the shop and it's only after a little time that they show signs of whitespot or other infectious diseases. Having them in quarantine for a period gives you a chance to spot these things before you introduce the new fish to the rest of your tank.

Some people will tell you to get one now, because it's the safe option. You can add an infected fish at any point and it could potentially wipe out all the fish in your tank... Some people have a lot of luck with being from the same shop(s) where the fish are well-looked after and quarantined - I buy from two shops only and (touch wood) have never had any problems from either. Fish I've bought elsewhere haven't brough disease with them, but haven't survived, so I stick to the places I've had success with.

It's up to you, but if you have cause for concern, and can afford to set up and run a small QT, then I'd say go for it. Peace of mind's worth every penny, in my book.

Quarantining fish is always useful, to protect the fish in the established tank from any illnesses/pathogens being harboured by new fish introduced to the tank (and which might take several weeks to manifest). Therefore, while the first batch of fish can be added to the main tank straight away rather than needing to be quarantined, it would be wise to quarantine future fish for several weeks.

How many fish do you currently have in the main fishtank and when were the most recent ones added? It's usually possible to safely move up to 1/3 of established filter media into a quarantine tank's filter, if say that filter media had been supporting a set number of fish for at least a month. Depending on numbers in your main tank, it might be that 1/3 of the media would happily support all 10-15 newbies at once in a quarantine tank - but probably only if you've currently got about 30 fish in your current tank and the filter media has been supporting that number of fish for at least a month. Hope this makes sense but, if not, let us know the answer to the question above and we can advise better.

What you would then do, on transfer of fish from the quarantine tank to the main tank, is also transfer the QT's filter media (+/- its filter) so that the established tank has all the filter media and thus beneficial bacteria required to support all your fish in there.

It doesn't have to be large, 25 litres is fine for fish up to a couple of inches. You would also need a heater and a filter - one that you can add some of your main tank's filter to. A couple of plastic plants to make them feel secure and a very cheap hollow ornament for a cave - I use sections of plastic drainpipe! Once the fish are in the main tank, put the filter media back in the main tank's filter and find a corner in a shed/garage to store the tank till next time. It also doubles as a hospital tank. Sick fish can be treated separately, which avoids unnecessarily treating all the fish for a sickness they don't have, and a smaller volume means less medication to dose. Of course there are some infections like whitespot where if one fish has it they all do, so you have to treat the whole tank but there are other infections that affect just one or two fish.