Setting up quarantine tank

[Wrench]

What fish do you plan to QT in a 20g? I've found that 30b and 40b tanks work well because the extra water volume helps with stability. For larger fish like tangs I use a 55g tank which helps keep the stress levels down. Medication can get costly though, which is the only downside. The larger tanks also work for multiple fish. A 20g would suffice if you're treating small fish.

[YHSublime]

I think a 20L is perfect. Of course bigger is nicer, but the footprint is just right, IMO. Besides, I don't think the OP will be adding large fish to their system, from what I understand.

[icsparks]

So far I have cycled 2 new fish through our 29 gal QT with no issues in water quality or stress to the fish that I can tell. The fish did their 30 day lock up just fine  with no apparent illness.    I do 5 gal WC every week. I had to place a container of sand inside the tank because one of the fish was a Yellow Cloris Wrasse. My only issue I have had is with the new resident. The new resident fish is a small blue hippo tang. For some reason this fish hides all the time when the lights  are off .When the lights are on the fish will be out and about  happy. So I kind of disagree with the "No need for lighting" I think the fish need some sort of a lighting schedule to feel normal and not be stressed out hiding in the dark all the time.