A quarantine tank need not be an expensive addition to your fishkeeping hobby, and could avert a potential disaster. A simple setup is described below.
A small (say 18x12x12") tank should be suitable for all but very large fish. No substrate is necessary as this will make the tank easier to keep clean as well as cutting the cost. Use some sort of backing on the tank to make fish feel more secure, this could be extended along one of the short sides of the tank. A 50-100W heater should be sufficient. Use a small inexpensive sponge filter (this could be left running in the main tank when not in use to keep the filter 'mature', and returned to the main tank with the fish once it has been established that they are healthy).
Keep decor to a minimum to aid cleaning (e.g. rocks, plant pot 'caves' or plastic plants), but provide a refuge for fish to feel secure. Without live plants it is not necessary to light the tank, this will also reduce the cost and the fish may feel more secure in subdued daylight than they would under bright lighting.