For a 40 liter, keeping a goldfish in it, (even temporarily) I'd put a pretty powerful filter with adjustable flow. Since water quality is SO important especially if the fish are sick.
You will want a filter that pumps at least 400 liters/hour or more. The Fluval and Eheim filters are very good, compact, but with lots of power and space for biological filter media.
An undergravel filter would work too, but then you'd need gravel for it to be useful at all, and most people rather avoid putting gravel in their quarantine tanks.