Here is an excerpt from a sticky I was working on:
Now you have decided to quarantine - what equipment is needed?
Tank:
The first thing that is needed is a tank. Now, the tank does not have to be large. It all depends on the size of the specimens that you are quarantining. If you have a large tank and get large fish, then go at least a 40 breeder. If you are quarantining smaller fish like clowns or firefish, then get a smaller tank. I recommend a 20 long as a good quarantine tank. However, the tank does not necessarily have to be a fish tank. It can be a Rubbermaid container (food grade quality). The key is that you need to be able to clearly see the fish to be able to observe it for any signs of disease. This would be difficult to do with a rubbermaid container.
Filtration:
I would recommend a hang on back filter or even a canister filter if it is available. I find it best to use a filter that is rated at least double the size of the tank. For example, your tank is 20 gallons therefore I would get a filter rated for 40 gallons. During the quarantine period, it is acceptable to run carbon. If at any point you need to medicate the tank, remove the carbon. Carbon will absorb the medication making the treatment useless.
Heaters:
An appropriately sized heater for the tank is recommended. You want to replicate the same temp for the quarantine as you would the DT. This will help make the transition to the DT a little smoother.
Light:
I recommend having a light for the tank. This will make it easier for you to see the fish.
Magnifying Glass:
This will make it easier for you to see parasites on the fish.