I'm glad you got your leak taken care of. Sorry you had to pull it all apart to do it. I hate plumbing....
The fishroom tanks are ... well ... to add water volume and to serve as refugiums of all different sorts. I feed a lot and need lots of nutrient processing. One of the 90s will be a RDSB. Another will be for chaeto. I'd like to turn one into an ulva and gracillaria tumbler to grow tang food. Etc.
Only one of them (my old 120) will be able to easily drain to the bottom. I'm using it as a water change tank (with a bulkhead in the bottom and ball valve to a hose I can run to a sink).
The other tanks are designed more to keep water in than let water out.

You can see one of the glass overflows on the top rear wall of the tank in the photo above. The black spa flex in the center rear of the tank takes overflow water to the drain line. The overflow from the top row of tanks feeds water to the bottom row (or, back to the sump).
The tanks are fed system water that's overflowed from the display tank in the main part of the house. In addition, I have ball valves installed so I can divert some (or all, if I ever need to isolate the display tank) of the return pumps' flow through this downstairs system. That way, if I ever need to isolate the display I don't have to stop the water circulation through the fishroom.
I am able to isolate any of these tanks individually, just by shutting off the ball valve feed to each tank. So, in the beginning of the system, I'm planning to use them as individual, large QT tanks. That way, I can purchase the stock that I want as it becomes available and still introduce the animals in a sensible order to the display. Well, hopefully sensible. We'll see.
Did I answer your questions?
After all of that, I realized that it would be easier to just label a photo: