First of all, always buy strong and healthy looking fish, but remember that even the healthiest looking fish could carry nasty parasites and illnesses. Therefor a quarantine tanks are hugely beneficial for fish-keepers and fish alike and any serious aquarist should have one available.
These setups allow the aquarist to monitor new arrivals for signs of any decease without the risk of an illness spreading in the main system. Should sick be present a quarantine tank allows the hobbyist to treat the individual fish or fishes instead of the whole system. A positive side that seems often be overlooked is the peace and quite a quarantine tank can offer new arrivals that might be stressed after travelling. The same goes for wild fish getting used to new foods and water parameters. For this reason I personally don’t stick to the bare minimum quarantine tanks but rather try to set up an environment that the particular species can feel comfortable in long term. I also try to keep the quarantine tank in a quite spot to minimize stress.
The most common villain I see is flukes, especially in wild fish. There are plenty of remedies for these and the fish usually recovers unless it’s a particularly bad infestation. I try not to treat the fish straight away and leave them to recover for a weak or two before starting any treatments, just to let the fishes settle in and gain strength.
As for quarantine period, 2-4 weeks seems to be a common recommendation. There is no need to rush and I personally prefer to leave the fish for around a month. This decision depends on the fish and where it comes from.
What you need is a suitable sized tank, a heater and a seeded filter as well as suitable decor for the particular species. Light are not important and if you have them they should not be too strong as this might put additional stress on already weak fish.
As my main interest lies in small sized catfish the tank in the photograph above is set up for these types of fish. The tank is 72 litre, 60x40x30cm which allows enough bottom space for small sized plecos. The abundance of caves and wood lets the fish pick and choose hiding spots. The lights are particularly strong in the photo, but I don’t use them at all for the first week or so and then slowly introduce the light.
This tank is permanently stocked with a group of wild guppies which keeps the system alive and healthy; it also features several spare sponge filters in case they are needed to set up a second tank. If needed, the tank can easily be re-arranged to cater for many different types of species used to similar water parameters. With tetras and otos I would usually add a number of plants for instance.
This is how I do it but there are many ways to skin a cat(fish)
