Were you thinking of having a rectangular tank made with the square section cut out on the corner?
If so, you might consider instead to just make a rectangular tank and have it be 'set back' slightly from each wall - sort of a 'porthole' look. Dressed up well this could look great and would be easier to build and more flexible if (heaven forbid!) you ever need to move it to a new location or sell it.
jayo
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