Here's the scenario. You've decided that you need to set up a special purpose tank - for breeding, for fry grow out, quarantine, or something else. Also, you want to do it in hurry for whatever reason. You have an extra tank that's just the size you need buried somewhere in a closet or the garage. Do you have a spare heater? Do you know where it is? How about a filter of some kind? Media or sponge? Do you know where those are?
Setting up a spare tank for any reason isn't complicated until you get started and realize you don't know where all the things you need are. I recently re-organized my walk-in fish closet. Sometimes as I buy extra components, etc. I bring the bag or box in and just set it in the closet rather than putting them where they go. I took care of that problem today. Part of what prompted the re-org was the need to set up a small fry tank (5.5 g) and the fact that I kept moving bags from one side of the closet to the other as I retrieved the supplies I needed.
Choose whatever organizational system works for you. As you can see in the photos, I like see-thru plastic bins but not very large ones. In fact, I have separate bins for the following:
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub