I will be storing my RODI water in a 40 gallon tank, but then again, that 40 gallon tank will also be my ATO resevoir, so it will be constantly getting used.
For me, the 3rd fish added was a blue Hippo tang whom almost immediately got ick... That night I tore the tank down to get her out and set up a quick hospital tank. Now she is fine with copper but the display will be vacant for 8 weeks. At this point I spent more money than I saved buying her on sale, and the fish are hidden for 2 months. I will be using a Q tank in the future for fish and waiting for A Red Sea Aquarium to get (and hold) the fish I want.
Quarantine Tank, I actually haven't really used one in the past but am beginning to because I don't want to risk my tank and it's at the point where anything that goes into the tank is very difficult to remove because of the rock work and corals.
The main reason I haven't run a separate QT in the past was lack of space.