Upfront, this is what the stand looks like:

https://i.ytimg.com/vi/XVkMbtOa0j4/hqdefault.jpg

And funny enough, the top of the tank is level (checked with water level) :razz:

Measure across the top of the closet that the tank is beside and then do the same at the bottom. If the opening is square the width of the opening should be the same, top and bottom.

Maybe your building skills is just 50%(or lower) of the problem.

I did but it appeared to be almost the same (probably 0.1 difference) but the stand looks way too crooked.

It is probably more than 50% :redface:

Actually to square something up you need to measure diagonally corner to corner.

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LOL probably a good call on the biocube stand then.
I like the plain simplistic look anyway