Kind of unrelated to the entire post, but if it is actually "sealed", you are likely going to need a gate valve to regulate return flow otherwise you will be creating a syphon etc. . .I don't understand what semantics you think that I'm stuck on...
I know that it's just more water. I'm not saying that it's special in any way.
I'm saying that "just more water" is what exactly I want, and nothing more than that. I can't fit a bigger sump in my stand, I've already maxed out my under-display space. So if I want a bigger sump, then I have to either build a new stand, or have a standalone sump sitting next to my display.
Let's say that I do upgrade my display and my sump. I build a new stand and I max out my new and bigger fuge, and I stock my display full of coral and fish and inverts.... And I discover that even though I've doubled my water volume, I've doubled the demands on my water parameters as well. So my stability is still not where I want it to be. Wouldn't any tank of any size benefit from increased stability?
Should I upgrade again? And again? At what point can I stop chasing bigger displays and bigger fuges and just add more water volume to the system instead?
If I'm happy with my sump as it is, and all I want is more water volume, it seems like a fine solution to just add more water volume.
What I'm arguing for, is a remote insulated sump, which uses gravity to return tank water to the main sump, with a sealed lid to prevent moisture escaping, all hidden inside a wooden box. That's all.
Does it make you feel better if I call it a sump instead of a container?