the current position is pretty perfect:
I would doubt that:
I can look at it from the sofa and from the dining table.
True, but it’s quite far away for that. I like to sit a maximum of 1-2m in front of it. I’ll even take a chair for that, if necessary.
It stands centrally without much "through traffic" in front of the window.
Well, it’s not that quiet after all, and it doesn’t necessarily need to be. Or what kind of animals are supposed to go in there?
It has no direct sunlight.
Why? The window is directly opposite, even southern sun.
The technology (e.g. large filter tank with about 500L) can go down to the basement, etc...
What will it be? Saltwater? And now to the disadvantages I see there: Walls on three sides – that’s bad. First, you lose the possibility to see the tank from the side, and second, it’s a pain if you have to get to the back corner. Cleaning the glass at the side bottom back will be a nightmare. If you need to get to the technology – bad again. Laying or removing hoses – you always need to be able to access from the side as well. I wouldn’t do that. If you really want to achieve the "built-into-the-wall effect," then operate it from the adjacent room (e.g. a storage or utility room) with a cutout to the living room. That’s pretty cool – like in a zoo.