This comes up again and again, but I personally never used to use a quarantine tank when I had just my 60L tank.
Basically at that point I was keeping basic (fairly cheap) fish, and didn't have many. If I put new fish in the 60L tank and they got sick it wasn't too much effort to treat the entire tank.
However I now have a 300L tank that is reasonably stocked as a community tank, and I now quarantine. This is because if I buy some fish and put them in the 300L tank there are a LOT more fish at risk than there ever was in the 60L tank, and also it costs a small fortune if I end up having to treat the entire 300L tank, where as treating the 40L quarantine tank is a lot easier, and also makes it easier to keep an eye on the new fish.
Saying that my 40L quarantine tank has now been taken over by cherry shrimp, so if I do end up wanting to buy more fish I'll need to find myself another quarantine tank - lol
At the end of the day only you can decide to quarantine or not, but personally for me it is a balance between the cost of a quarantine tank and the stuff for it, Vs the cost & upset of losing the fish you already have.
I don't think I would recommend quarantining in a dark cupboard though. I don't think the fish would appreciate being shutaway for that length of time, and they wouldn't appreciate then being moved from that environment into the main tank. Part of the quarantine process is to minimise the stress of the fish, and going from several weeks in darkness, into a 'main tank' would be quite stressfull i would think!
